I had something like this and then more lumps developed. What felt/looked like a painful zit or spider bite became several carbuncles under my arm and on my inner thigh. It was a staph infection and had to be treated with an antibiotic. I hate to say it but I think I got it when my MIL was staying and she may have borrowed my razor.

I get these sometimes and they go away after a few days of being extra careful with washing well with soap at night, let the skin "breathe" until I have to put deodorant in the morning, and also changing deodorant brands.