could be a small cyst. Could be caused i think by a repeated activity with that area and will go away if not used repeatedly in that way. But you also may want to get this checked out may require surgical intervention
Hello if you are sure you've not stuck anything in it is most likely a ganglion cyst. They are very common. Other causes could be a subaceous cyst, calcium deposit, arthritis, abscess, or tumor. Since you are having pain you probably need to check with your doctor to ensure it is not something more serious. I Hope This Helps
If you dont remember having hurt yourself recently, and if the bump hurts a lot, then you should meet your GP immediately.