I'm not a real doctor (as my kids often say).
Entirely possibly it's a sebaceous cyst, which is just a little fat pocket under the skin. Sometimes they happen and are completely dormant, then begin to fill with puss and will either balloon up or ooze.
I had one right in the middle of my chest, when I was a teenager it showed up randomly, looked like a tiny BB under the skin.
Fast forward 10 years, it randomly started growing, over the course of a few weeks grew to the size of a grape. Dermatologist lanced it just to get the nastiness out, but had to remove the underlying pocket (like grape skin) to prevent it coming back. Left a huge scar b/c it had grown so much.
Also had one on my wrist once, that one was just scalpeled off. No problems since.
A dermatologist will deal with this properly. Unlikely to be anything serious, but if not dealt with could mean an issue/scar later.