What's wrong with my wrist?

I was at my tennis lesson this past Monday when all of a sudden my hand started cramping really bad. It only hurts when I pick something heavy up or If I am serving a ball. It hurts where my thumb connects to my hand. It's not swollen or bruised. Just sore. Any ideas? I've been using an ace bandage, ice, and heat pack. Thanks... Oh! I also have a match on monday; so please help! lol