pain in thumb joint

hey i've been [playing a lot of tennis recently. when i start hitting/warm up i don't have too many problems. but after i hit a big forehand or soemthing, the joint inwhich my thumb connects to my hand start to really hurt. its a sharp pain whenever i hit the ball, especially when i follow through over my shoulder on my 2 handed backhand, and then make your hand into a thumbs up with your thumb pointing up, then without rotating your hand bend your hand towards your body. when i do that i get a really shart pain if that motion made any sense lol...help!
 
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