I have a lump on my armpit it feels like a little ball or marble under the skin. i have had it for months it was small but now bigger and painful.

Lymph node: You could have an infected lymph node or infected sebaceous cyst. Either way sounds lie you need to see a medical doctor. Keep warm moist compresses on area and use ibuprofen until you can get in.

Swollen lymph node: Due to the location and signs of swelling and tenderness. Most likely is a swollen nodule and will go down with some anti inflammatory medication. If swelling persist for 7 days you might need to have it checked at your PMD

Sebaceous cyst or: lymph node which may be infected so you need to see your doctor and have it checked. Warm compresses may help.