late period + negative test+ morning after pill + sore breast = ?

My period is about a month late. I have taken two pregnancy tests a week apart from each other and they both came back negative. I had unprotected sex and got the Plan B pill the very next day. My breast are suddenly extremely tender....I'm confused with it... I don't know if they are sore because I could be getting my period, if I'm pregnant or is it possible for the pill to make them feel that way. Opinions and advice would be much appreciated!! PLEASE!