can anyone tell me what this is

It's on my armpit. And it hurts. I don't have it on my other arm pit. I've felt it for a bit I think. I just now looked at it. 
 I don't know how to answer anonymously, if is not itchy. It just hurts. It's starting to lessen. My husband says it's a brush burn. I noticed it right after I showered. Usually hurts after I shower. :(