You did the right thing by seeing a doctor to evaluate the masses that you feel in your arm, breast/rib cage, and lower back. You are correct, it is impossible to tell with certainty what the masses are until you have them excised and the pathologist looks at them under the microscope. However, lipomas are common and have consistent characteristics which makes the diagnosis more likely. The lipomas usually are mobile (not stuck to underlying structures), soft and have no overlying skin changes. More worrisome masses are hard and stuck to surrounding structures, including your skin. Your back pain may be due to compression of the lipomas or change in posture from adjusting to the mass on one side of your back and not the other. The lipoma may well be causing your pain, but you can only know for sure with a complete evaluation by your doctor. You may also have pain for some other reason that is not identifiable.