Small, hard, almost egg shaped lump under the skin on far right of back, just below rib cage. no pain. there a couple months. any reason to worry?

Always worth a look: At 16, it would be extremely rare to have a "bad" mass like a cancer, but there can be things like sebaceous cysts, small abscesses, ingrown hairs, lipomas, and foreign body granulomas....So if it becomes painful or larger, gets reddish, or changes color, or if you have the time while you're off this summer from school, i would recommend that you have your family physician or dermatologist see it.