Help! False Negative Pregnancy Tests?

I was supposed to start my period four days ago (Tues Sept 29th) and haven't yet. I have taken two pregnancy tests, one Tuesday, because I suspected I might be pregnant, and one Thursday. Both tests were negative. My husband and I have been having unprotected sex for about a month, and a couple times during the suspected ovulation time. Why has my period not started if the tests are coming back negative? I am 21 and in good health. What do you think the probability is that we are pregnant?
Also for the last two-three weeks I haven't been able to fall asleep at night, which has never been a probable for me.