Years ago I found a bruise that came out of nowhere on the top of my forearm. Over time it developed into some sort of large grape sized lump/bump which is painful to the touch. You can't see it protrude on the surface unless I move my arm in any way. If I flex my hand down it really shows through and it feels like it is deep in there in my muscle and ligaments. Anyway, the plastic surgeon I had worked for who's also a trauma surgeon looked at it briefly and said it was a lipoma. My ortho who has done my two knee surgeries (non-related to this, obviously), looked at it today when I brought it up to see if he knew what type of specialist I need and said he didn't think it was a lipoma and requested an MRI for me this week so he could see what it is and refer me to the appropriate doctor. 

What else could it possibly be? If it is attached to ligaments and muscles, etc, could this be a more complicated surgery and is it possible to drain whatever it is or do a biopsy without surgery to test it in any way? I'm curious if this is anything I should be concerned about. There's a lot of various cancer in my immediate family with related issues and other types of nodules and tumors, etc on both sides for many generations before me and after me. 

Any ideas?