I have been experiencing severe pain in my right thumb for the past two weeks. It's a sharp, stinging pain along the inside portion of the thumb itself, mostly near the pip joint. This pain only occurs when my arm is fully extended and when I cross my thumb toward my pinky or when I rotate my thumb down toward the ground (like I'm giving a thumbs down sign). Reaching into my pocket causes pain as well as reaching for anything that is far enough away that my arm has to be extended. There's no swelling or redness and my grip itself hasn't diminished at all either. If my arm is bent then I can rotate and grip things fine with no pain whatsoever. There is slight pressure/pain when I squeeze my thumb but not nearly as much as when I reach for something. Do you have any clue what could be wrong?

Sure - you pinched a nerve somewhere in your arm/wrist. Which nerve might take a ton of work to find, so generally a PT or doctor will prescribe warm tub or whirlpool, gentle exercise, moist heating pad, and some Tylenol. If you sit at a computer a lot, using that hand on the mouse, then you need to adjust your position. Most all of us flex our wrists to the side unnaturally. Make sure it's straight in line with your whole forearm, and make sure your shoulder is relaxed, not hunched up.