I have a non-painful lump on the outside of my right bicep. It feels deep and as big as a marble. It is about 2 inches above the elbow. When straightening the arm it is half way between the inner elbow crease and the elbow. It feels kind of mobile but I am not sure it may be immobile. It is hard. It is not visible on palpitated. I noticed this in the last six months. I have had some muscle quivering in that arm occasionally but not lately. What could it be?