A quick physician's office visit would determine it. Without seeing a photo, sounds like a ganglion cyst or "bible bump". Bible bump, being old schoolers used to used to smash it with a book as thick as a bible. Not recommended. I had one on my wrist which was punctured and drained by a physician - it came back - finally went away with "pop" while doing handstands on the beach. A little ibuprofen/beer on the spot and it has never come back (5-6 years ago).