I Have An Unusual Round Firm Bump Under The Skin

I have an unusual round firm bump under the skin in the palm of my right hand, which appears to have a half inch "stem" radiating toward my wrist. It is not tender, but appears to have gotten more pronounced since it appeared about a month ago. I have arthritis in my hands and fingers. This does not look like a callous nor a bruise. There is no marked skin color change. I would appreciate your thoughts. I am a 68y/o white female.