Why are my nipples so sore?

Both of my nipples are really really sore but not my boobs.. I have sex with my partner regularly with no pertection. I took a pregnancy test and it was negative.. Its only been a couple days but they REALLY hurt! I have never had this problem before but I was told that it could be because im about to start my period? BUT.. why has this never happened before? I cant remember when my last cycle was but I think its pretty soon.. any ideas??

Dear steph2005,
First of all since you mention no protection??? Why...are you trying to get pregnant and get STDs? Always use protection. The reason you are getting sore is from a hormone surge. This is typical before your period starts. Very typical for many females.
Sue...good luck