There are many problems which can occur with improper use of the mouse. First, you are most likely bending or twisting your wrist (ulnar deviation, radial deviation). Just because you can, doesn't mean it is okay to do. If you take a live branch and bend it left, then right, then left, etcetera, eventually it will develop cracks and maybe break. That is what you are doing to the tendons and the tendon sheath in your wrist when you overuse and misuse your wrist in that way. Go write on a chalk board and notice that your arm places your hand where it needs to be. You need to move the mouse with the same unified gesture. It is one movement without isolating the fulcrum of the wrist. The movement comes from the elbow, shoulder and back, and is counter balanced or supported by the stomach.
Second, you may be sitting too high or low. The wrist not only has to be straight and not move left to right, but, the wrist shouldn't bend up and down, either (dorsiflexion, flexion). If you sit too high your wrist will have a bend in it. Sit too low, your wrist will have a bend in it. Your hand to your elbow should be a straight line, like writing on a chalk board. Many people will unconsciously try to straighten the wrist by either slouching or raising their shoulders. This will literally give them a pain in the neck.
Third, people rest their forearm on the table when they use the mouse. That can cause a bend in the wrist and it can also add pressure to the carpal tunnel. If your tendons are already inflamed and pressing on the median nerve, this added pressure or resting weight can further press on the nerve. The arm should hover and not be lying flat. You couldn't write on a chalk board if your arm was pressing on the surface. You've got to find the medium position where you are resting up. That may be tense for some people but if you are always moving and also resting down, the constant movement creates a sense of rest and ease. If you sit in just one position, that is called static loading and you can create stress issues from maintaining a single position. If you were to move your arm rapidly up and down as if you were spanking a baby a dozen times, your arm would fatigue because you are only using two muscles to go up and down. Also, chances are by the time you start to go down, your up muscles are still being employed and this is called a dual muscular pull. That is what causes tension. Try it (not with a baby). Now "spank the baby" again but this time instead of going up and down, slowly make a circular motion with your arm. You shouldn't feel the tension in your forearm at all because you are now using dozens of muscles to go up and down and left and right. By the time the arm moves up, the down muscles have already relaxed and rested. If you feel any tension it may be in your upper arm or shoulder. Those are different issues. You may also feel tension in your pronator teres muscles so to properly spank your baby, have them stand up so your arm and hand is vertical to the floor. Dangle your arms to your side. That is their natural position. Now raise them up to 90 degrees with the palms facing one another. You should not feel any tension. Now pronate them or turn your palms so they face the floor. Feel the tension? Now supinate them so that the palms face the ceiling. Feel the tension?
Fourth, depressing the button on the mouse isolates the forefinger. This is very bad and can lead to long flexor tendonitis. Tendonitis is the inflammation of the tendon and when it inflames (swells up, gets bigger) it will press on the median nerve in the carpal tunnel within the wrist because there is no extra room in the tunnel for expansion. Your fingers are designed for poking, not lying flat and pressing down. Go ahead, lay your hand flat on a table and using just your forefinger, press down with all your might. Feel awful? Do you feel it radiate into the forearm? Now lift your arm up and with naturally curved fingers, using the arm and without pressing, gently poke the table. Effortless? When the phalanx, finger joints, knuckles, wrist and elbow are all aligned properly, they have tremendous and effortless power (not the same as strength). The best way to to press the button of the mouse is to gently hold it with your thumb and pinky, not squeezing, you are hovering, and you forward shift into the button. The button is pressed from the elbow and you are NOT ISOLATING A SINGLE FINGER. As you forward shift, the whole arm, hand, wrist and five fingers are moving forward and you are using a very slight adjustment of the forefinger to press the button. When you press it the old way, your are isolating the forefinger and since all your tendons are interconnected, you are creating a dual muscular pull, which is bad. Try it with a doorbell or elevator button. Isolate a single finger and press the button. Then with all five fingers together, using your longest finger, press the button from the elbow. Effortless? I am also barefoot 90% of the time so that prevents ankle and foot fatigue and allows the ankles to do their job of micro adjustments for balance. I also stand on four of those polyurethane foam bath mats for a nice cushioned and arch supporting feel.
Teaching yourself to move properly can be awkward in the beginning but once you set up your "work station" to the proper ergonomic setting for your own body, and then employ the proper movements, you will heal. My lap top is actually on a small table on a table. I stand in front of my screen which is slightly higher than my head so that my back and neck are straight and stretched a bit. My wireless keyboard is at waist level and angled so that my arms can dangle at about 135 degrees. My wireless mouse is elevated at 90 degrees so my whole arm is free and can move effortlessly. I can stand at my computer all day because I am in constant motion and there is zero static loading. I also have a small padded stool in front of me at knee level in case I want to pause, I can lean on the stool with my knees. There is also a bar stool behind me for moments of pause when I am reading something or don't need to use my arms. My mouse pad is also on a clipboard which I sometimes pick up and hold to my stomach and I move as if playing a guitar. It is the constant rotation of movement which prevents stress, fatigue and strain to my muscles and tendons.
Since some people have thoracic problems from slouching because they look down to their computers, they should take something like a Feldenkrais class to learn how to move the rest of their body properly. If you stand on a garden hose, the water pressure will diminish. If someone else stands on it a few feet away, it will diminish even further. That is what happens to a lot of people. There is a "kink" to the nerve at the neck and they are kinking the same nerve in their wrist causing a "double crush."
Not everybody has the mental acuity or patience to heal. Because of this, healing is easy for some and difficult for others. Most likely a doctor won't heal you if you have a problem. They treat symptoms, not problems and the problem is probably your movement. Many people will opt for surgery if they have "carpal tunnel syndrome" or, median nerve entrapment. The surgery just provides more room for the symptom which is inflammation and, does not address the real issue and cause of the inflammation in the first place - which is improper movement.
Drugs, splints, braces, rest and physical therapy only treat the symptom and not the problem. They can actually cause a further downward spiral of symptoms since they can alleviate pain, which is a symptom. If you start to feel better because you are treating symptoms or taking anti-inflammatory drugs, you are still moving improperly and you will feel good only until the inflammation gets worse and surpasses the temporary alleviation of the symptom. Pain is good, it tells us there is a problem. Fix the problem, not the pain.