Welcome to the forum.
what you mentioned about sounds to me like a cyst
, but cysts are generally with pain. Well it could even be a an infection/PVNS(Pigmented Villonodular Synovitis
).Hope you havent met with any injury
in recent past, for it could be a hematoma
too due to the same.Whatsoever, you need to get an X-ray taken, and if not an MRI as all pathologies do not show on an X-ray.
I hope this information has been both informative and helpful for you.