I had showered after doing a light workout. I happened to look in full length mirror in dressing area and noticed a walnut-size lump under the lower left bone of my rib cage. I had never seen this before. Don't know when it developed. I checked the right side and nothing. The lump feels hard and rubbery. Anyone else ever had one of these? I am rather thin but could it be some kind of fat deposit? I have a lump in one forearm that the doctor says is tendinitis., but I would not expect tendinitis below the rib cage. Do you think this is worth a trip to the doctor?