If you are used to getting your period around the same day every month, perhaps you may have forgotten that February is a shorter month? Did you count 28-30 days after the end of your last period?
Otherwise, drastic changes in your diet or stress can easily cause problems. Just wait a bit, and do another pregnancy test.
Also, if you are sexually active, you should be in touch with a doctor you trust, and try to get a PAP test annually just to be on the safe side.

Here is some info:
http://en.wikipedia.org/wiki/Pap_smear