What Does A Painful Lump On Bicep Indicate?

I had a lump come up on my bicep about a week ago for no apparent reason. It is about the size of a golf ball. It is very painful and hurts to touch and when I move my arm away from my body. It hurts all the time but sometimes more than others. Had not done any heavy lifting or much of anything in days prior to onset. It seems to be on the bone and not the muscle but not sure. Don t have any less use of arm it just hurts real bad when I use it.