What Causes Delay In Menstruation And Sensitive Breasts?

Hi, I have been feeling like I m coming on my period for the past week. My last on started on 18thdecember. I am usual late anyway but this time it s been longer. I am still letting has out a lot and have sensitive breasts and stuff but nothing is happening. Me and my boyfriend have ha unprotected sex but he is 99.9% positive he has never comes inside of me. By the way I am 18. I am scared to death. Help me!