What Causes Missed Periods While UPT And HCG Test Is Negative?

Hi. im 25 years old, unmarried. i had my periods on Jan 27, 2014. Then on Feb 22, 2014 i had protected sex but i doubt little bit of semen was on latex the second time. I did not get my periods on time. I did around 1 urine pregnancy and 1 blood HCG test (HCG level being My usual period cycle is 27-32 days. Usually my periods are very regular 1st time I have missed.
Am I pregnant? SHould I take this tablet? What can be reason for this missed period?