5 days late, sore nipples, and negative test

Guys help me out in what's happening here. My husband was back from the Air Force during Christmas break. The last time we had sex was like Jan. 1-2 and I got a period about 2-3 days later and lasted my normal 5 DAYS. And now I'm an entire period week behind schedule from my period this month (fen), my nipples are sore, my boobs are swollen, but I have a negative pregnancy test. What's up with me???