Re:Lump behind nipple?
Hi mate, I had the exact same thing recently, saw a doctor (about something else) and mentioned it and he said it was most likely [I forget the official term] small infection, the sort you'd get from an in-growing hair. I left mine alone and it went away eventually. I had an attempt at squeezing it out to get the puss out but was too deep and too painful.
I'd imagine it is that but if in doubt, see the doctor.