You could have simply hurt yourself in some way. Perhaps you "zigged" when you should have "zagged" and asked something that your wrist couldn't do for you in a match. Perhaps you reached a little too far, had too much of an extreme angle between your forearm and wrist and you put a little bit too much pressure on the ligaments in your wrist. You may not have noticed it at first, but now the pain is getting to you.
Perhaps use the RICE method of rehabbing your wrist and take some time off of tennis?