Why does my armpit itch so badly?

I’m so glad you asked this question because mine has been itching off and on for several months! I’m beginning to suspect my deodorant though, because I got some in my stocking from my mom that isn’t what I normally wear. I hadn’t used it and then packed it as my to-go on a trip. After that I was switching back and forth between the two since they were both open and recently got to wondering if that was the cause. So far since I’ve stopped I haven’t itched. We’ll see if it comes back.