I found a bump, why is it there?!

I found a bump, why is it there?

I told my mother I found a bump on my left boob. I got kind of scared because I don't know why I got it. My mother says that, it's because I'm on my period and that its normal because of hormones and that I'm still growing. I was wondering if its true and if anything like this happened to any of you [girls].

I think you should just check with your doctor... to be on the safe side.