Hi - This sounds like a skin condition of some sort. If it was on your breast rather than under your arm, I'd say keep your eye on it and if it appears to be getting worse - if your breast appeared to be getting infected (swollen, red, hot, itchy, painful), then see a doctor ASAP, to rule out inflammatory breast cancer. But as it's under your arm – and even though breast tissue CAN stretch from the breast up into the armpit – I'd say this is probably an allergy or hormonal issue of some kind. Nonetheless, if it doesn't abate in a few weeks - or if it gets noticeably worse – you might want to see a doctor to get it looked at. Good luck - PJH