Pregnancy scare, tired all the time.

Hi,

I had sex on 12/31/12 and had a scare the next month. I took three pregnancy tests that month and they all came out negative. I ended up getting my period that month and it was heavy. Then this month, (February) it's kind of late. I think. But I haven't had sex since that night. I took a pregnancy test this month and it came out negative. The only thing I have that's a symptom is tiredness. But i don't know if that's PMS or not. Please help, I'm super worried. :(