Super hard nipples, so hard they hurt.

My nipples have been hard and tender for 3 days. It's just my nipples and not my areoles, there is also no visual change what so ever in my breasts at all. I think it's from a bra I was wearing that is much too small, but I'm not sure. I just finished ovulating and my period is in 12 days. Does anyone have any advice as to what the cause could be or how to ease the pain. The pain level is at a 4 but only then I touch them. It's not present when I don't. (The right also hurts more than the left) I practice safe sex with only one partner and I know I'm not pregnant. Any other cause you could think of?