Why do I have a lump under my armpit?!

I have a small, hard pea sized lump under my armpit. It's sore when I touch it, and I had one in the same place a couple of weeks ago. I'm 14 years old, female, and I haven't started my period yet. I don't really want to tell my mum, but I'm scared, because everywhere I've looked, people say that it could be a blocked sweat gland, and I might have to have an operation :s Does anyone know what it might be?
Thank you!