hand suddenly changes position? Arthritis?

I don't know if this is arthiritis but Im 16, and when i write before long my hand starts to really hurt. I try flexing my fingers out and than continue but the pain doesnt go away. And than recently this last month its gotten worse. Now sometimes when i write alot something inside my hand suddenly changes position in a way that i know it shouldnt. It only happens for about 5 seconds but during that time my hand not only hurts alot more, but i can barely move my thumb and pointer finger, and you can clearly see in the area between those two fingers an indentation where something (not really sure what) has shifted to the wrong position. When this happens i usually just try to stay calm and hold it with my other hand until it goes back to normal. This has also happened a couple times when i try to reach behind my back to unhook my bra. So far its only happened around 6 times but im afraid it will get worse?