Mary-Ellen, an inverted nipple doesn't cause breast cancer. It can be a symptom, so when it appears suddenly (unlike in your case), it's checked and biopsied. But it's not a cause. Mastitis is a breast infection, and I imagine it's possible the biopsies could have introduced some bacteria into your breast, so it's possible these two things could be related. As to why it's not clearing up, sometimes these things can be very stubborn. I'd say be sure to advocate for yourself with your doctor; the the squeaky wheel, tell him the antibiotic isn't working and (s)he needs to figure out some other treatment. Good luck to you - PJH