There is something on my skin and i have no idea what it is!

I have been having this big, red boil sort of thing on my armpit exactly where the shoulder skin joins the armpit. It hurts a lot, i mean, i cant move my hand freely...and the skin around it is a little hard.
I used an old razor few days back and it may have been rusted...is it coz of that?? is there any cure for this thing???