I had a ball like that on 1 side.
It was a gland not working properly.
Sometimes they go away. Mine did but then came back and was painful to sit so had it drained or removed. It can be done under a local if its not too big ir a general if its a big 1 or an old 1 -had it a long time.

Sent from my Galaxy with the barefoot princess covering me in kisses, so please forgive the mistakes