I know that after I haven't hit for a few weeks and I get back out on the court, especially if it is for more than two hours, my hand will sometimes get stiff and it will hurt when I'm tightening my grip to hit the ball. To me this is natural, it goes away in the next few days while still playing and I believe it's just my body not being used to hitting such a huge amount of balls. If it's hurting even when you're off the court this is probably something different, but otherwise I think its just your body not being used to it. When
I first started playing competitive tennis my wrist was really sore. You should try to build up your forearm muscle and focus on hitting the sweet spot to decrease the amount of vibration going through your wrist. I hope that helps!