Could my irregular period be why i am 6 days late?

Hello,
I have a problam that most of you women out there have. OK so heres my question.
Last month (january 07') I started my period on the 7th way to early then i usually start ok well the same month i think on the 14th i had sexual intercourse (using condom) and now its the 13th of febuary and i am 6 days late for my period (considering i would start around the 7th again), Ive taken 4 home pregnancy test and 1 at the health department (Urine test) which makes a total of 5 NEGATIVE pregnancy test! I have been having little period cramps and irritability like i usually do before my period but i still havent started yet. Could i still have a chance at being pregnant? Or is it maybe the stress and anxiety of possibly being pregnant thats making my period late?

I used first respones on all 4 home pregnancy tests

Please help im very scared and stressed!!!!!!!!