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
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
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