What Causes Red Bump Like Pimple Under Upper Arm?

I have a small red bump almost like a pimple under my right upper arm? Should I be leary of this? I put some alcohol on it hoping it will dry up, but it's still a little red, but not pussing or anything like that, but it's a little sore to the touch like a pimple but I'm thinking it's not since there wasn't anything in it to pop it. It's just a bit sore and red.