I have a rash on one armpit, its little red bumps with heads and there is one bump that is very swollen even under the skin. it is painful and itchy?

PAINFUL ARMPIT RASH: IF IT HAS BEEN THERE FOR AWHILE SEE YOUR DOCTOR. IT COULD BE SHINGLES.

This Is The Pits...: You may have the beginnings of an infection in that area. If it gets too advanced it could become a real problem that's hard to get rid of and at that point it would be called :hidradenitis suppuritiva" which means a recalcitrant infection of the armpit(s). Best to get it looked at and treated right away. It's not likely to go away on its own and that big lump is a sign of that.