Reasons for sore nipples?

For the past couple days I've had pretty sore nipples. I don't believe I'm pregnant, it is a possibility. We have decided to stop TTC for now because of moving and being stressed but we did have sex during my fertile week so of course pregnancy is a possibility . And of course since we had stopped trying I wasn't tracking ovulation. According to glow I'm a week past my fertile week so of course as I said it is possible for me to be pregnant but to early to test just wondering what else could cause tender nipples? it's not the whole breast just the nipples