Hello,
Welcome to the ehealthforum and I am really glad to help you out. Your concern is regarding the bump that you have in your armpit . This can be due to a hematoma formed under the skin due to trauma, a cyst or a benign skin growth. Apply warm compresses and take some oral anti inflammatory analgesic. If the symptoms are severe then it is advisable to consult your doctor and get a CT scan/ MRI done to confirm the diagnosis.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care.