What Causes Delay In Menstruation Other Than Pregnancy?

Hi, My girlfriend has been on the pill for about 7 months now and has pretty regular periods. We had unprotected sex two fridays ago and I stayed inside. She was supposed to get her period this past friday and she didn't get it. We have been very worried and on Sunday she took an ept and it came out negative. I know that is fairly soon after we had sex, but she used an early detecting test. However, she still didn't get her period. She told me that last night (Tuesday) she did have a very small amount of blood, but she wasn't sure if it was actually her period. She has never missed a pill and takes it at the same time every day. So between the pill and the negative test I feel like I shouldn't worry, but I can't help it. Is there a chance that she is pregnant? What should I do?