Lump under armpit

My sister noticed a small lump under her armpit several months ago. She had her doc look at it, she felt it and said its a cyst. That was that. Now last night, it started becoming painful, and she felt it and she said it was larger. Its under her skin inside. Anyone ever have this? Shes nervous its cancer, but I read cancer lumps under the armpit are typically painless. She is calling a dermatologist in the morning to have it either looked at or biopsied. Any experience with this?