My 14 year old athletic daughter has a pea sized lump that is painful underneath her armpit, any clues on what this might be and what i should do?

Hard to know?: Has she had any other symptoms? recent viral syndrome perhaps? One possibility is a "reactive" lymph node, or node housing white cells responding to some infection (probably very minor in nature, otherwise). Its painful nature and sudden appearance are consistent with this theory. Would expect resolution in a few days-couple weeks. If it does not, deserves medical evaluation.

Likely a lymph node: There are hundreds of lymph nodes scattered throughout the body which act as sub stations that sift through body serum removing germs and debris. These enlarge and react to localized events like bug bites and skin rashes but can also swell during generalized illness.Occasionally they enlarge to walnut sized and require drainage.Most just do their work and shrink back down within a 3 wk period.

See your doctor: There are a number of possibilities. It could be an inflamed lymph node from infection or a nearby scratch or bite. It could be an infected lymph node. It could be an abscess. It could be a cyst. It could be an ingrown hair follicle. It could be a skin infection. I suggest getting it evaluated soon in case she needs antibiotics or blood work.