The pain is sufficient to cause the use of a "sling". You may not be able to pick up any substantial weight. You may see a doctor and obtain a "diagnosis" (perhaps an incorrect one).
Here you will get the correct diagnosis and more important what to do about it.
Mouse-itis is similar to the pain experienced by neophyte piano players. It arrises from rotating the forearm 90 degrees out of its default position and keeping it there for long periods under some amount of stress. The pain results from the nature of our muscle structure. You can't for instance hold your arm out straight and keep it up indefinitely. It will start to hurt and you will be forced to lower it no matter how strong you are. We can't "lock" our muscles into a position with no further energy consumption. We derive our strength from motion-change-action not from stationary holding. (You can investigate the chemical reasons on your own. That's not important to this discussion.)
For the piano player the answer is to lift the elbows up and out so as to rotate the hand by arm position rather than rotating the wrist, i.e. the default position of the human hand in this instance (and for computers) is that taken when using a "joystick" and by lifting the elbows the hand is turned to the "piano playing" position.
But you may find that lifting the elbow is not practical for "mouse playing". It's awkward.
My answer (this corrected the problem for me) was to obtain a trackball (I have a redball Logitech) and mount it on a metal bookend 'sideways' with some stickon Velcro tabs (not exactly 90 degrees but about ~75-80). It looks like this and cost me about $3.00 at Walmart.
By doing this, my "problem" cleared up in abut 2-3 months. Yes, that's a long time but I continued to use my mouse out of necessity (you can't efficiently draw or drag&drop with a track ball). I simply navigate with the trackball, do file maintenance,etc. ... whatever I can with the trackball ... By dividing tasks between mouse, trackball and keyboard one can give the forearm that all important rest and reset time needed for "muscle maintenance". I can live with it now. I don't have to quit working alltogether as I once thought.
Another helpful and cheap thing to do is obtain a "wrist-rest". BUT NOT FROM THE COMPUTER STORE ! Go to Walmart, Kmart etc. to the automotive section. There you will find some sponges for washing cars (about 2x4x8 inches) and with a chamois covering. This is a wrist-rest. Use this and you will understand me. (You can also wipe off your screen & computer dust with it.) I've had one for a year or more and it's x-cellent.
& don't scrimp on a chair. Spend a buck. Get the best you can. (Same as you would for a bed or favorite easy chair)
& try putting your junk in a wire dish rack (plastic coated) ... pencils, pens in the spoon-fork cup. Works great ... there's more computer stuff in Wal-K-Mart than you think. Look around for solutions.
Use your own head.