I am presuming no other changes that concern you here - you know the drill - discharge, inverted, lumps and bumps etc. ?
Here's a thought, I had this once for weeks on my left breast; I eventually found out it was the seam on a lacy bra pushing into the nipple all day. Try wearing a seam-free bra/and/or put a dressing pad (non sticky, just a dry large one or even layers of tissue) over the nipple in the bra.
Worth a try?