Oh dear.. u've got TENDONITIS..
Its common amongst drummers, who plays without proper warm up and stretching. :?
The condition will worsen if U have a long period of session and U still persist in playing. Let me tell Ya boy, ur forearm will be immobile!! And I'm not kidding!!
Its a good practice for me to warm before every jammin session or gigs.. Be it on my way there, inside a bus or mrt(u'll be getting a lot of stares from other passengers but who cares. Its ur hand. Not theirs.) or come a bit early just to warm up..
An easy way to prevent tendonitis is to stretch those muscles and joints that are being used by our limbs for drumming..
How??
Hold and grip ur sticks. Twist ur wrist clockwise slowly to feel the stretch at the wrist joint and hold. Count up to 5 and return bak to normal position. After 3 or 5 reps, do the same repitition but this time anticlockwise..
This is just a simple stretching that helps to reduce tendonitis. There's still a lot more hand exercises that U could do.
Hope that I answered some of ur questions..