You should be going to a doctor, and not asking random people on an online gaming forum.
Anyways, does it feel like the bone, or the muscle? If it's the muscle, it could simply be due to overuse/fatigue of the muscle due to the repetition of serving.
Or, if it's the bone, it could be a hairline fracture, if it really isn't bothering you that badly.
I really don't know, you didn't give enough info.
I doubt it's a hairline fracture, or even that it's any sort of fracture at all. I'm guessing it's just fatigue of one of the muscles you're using while serving. Give it a few days rest, and you should be fine.
