my nipples hurt for some reason

My nipples are really tender and I don't know why. I haven't had sex in months, so I know I'm not pregnant. I was on the depo provera birth control shot, but I got my last shot months ago and havent had a period in almost a year. But my nipples never hurt when I did have a period. I'm not to worried about it because they're just sore, nothing else. I'd just like some advice on it please.