What Causes Scaly Bump On Calves?

I am a 53 year old female and last summer a bump came up on my back right calve. I thought it was a bite at first although a little big. It grew really fast within a few weeks. It changed quite a bit kind of scaly and just a little ulcerated looking and then it would look better then worse. I have watched it and wondered what it could be. It kind of has a ring around it too like the skin is turning darker. Went to dermatologist he said it was likely a dermafibroma but not sure I am happy with that. What do you think?