A ball under your armpit can be something as simple as an ingrown hair which is causing swelling, I get those sometimes and they will develop into a cyst, as the white blood cells are rushing there to repair the problem I wouldn't worry about something terrible like cancer just yet, the are glands in the armpit much like the one in your throat and neck which will swell when some type of infection is detected, like a cold, flu and as I said, in your armpit, may be something like an ingrown hair which often happens and sometimes can be recurring, there are these little stubborn ones who keep curling inwards. If there is pain associated, the your gp. Doctor and have it checked out. They will determine if it is serious and if you need a biopsy, or if drainage of the fluids is required. The more you worry about something the worst it gets in your mind. If you have strong concerns, of if condition persists or worsens, see a doctor as soon as you can. Like I say, if it gets worse, don't put off and have it checked out
