Sore nipples

Hello! I’ve never experienced any soreness in the nipples or breasts before with periods. I’m only an A cup and I think that might be why. Lately my nipples (only nipples not breasts) have been super sore but I no longer get periods. I’m on the nexplon and haven’t had a period for over a year. Anyway I figured I should get a pregnancy test just in case but that was negative. I’m just curious why this might be happening. I’m 20 now so I don’t think they are growing anymore but I’m not sure.