First time ever to have painful nipples

I am 31 and my nipples have been hurting for about a week now. But today they are the most painful! o.O Usually before my period my breasts are sore, but they aren't - only my nipples. There are no visible signs of change (no dryness or cracking). I was supposed to start my period today but didn't. I have taken a few pregnancy tests already (detect 5 days before) and all have been negative. My husband and I don't use birth control except for rhythm method (avoid days of ovulation). I had a miscarriage about 6 months ago and my cycle has been regular for the past few months. My nipples weren't sore when I was pregnant last time- only my breasts. I don't know why my nipples are so sore?? Does anyone know??