When did you take the pregnancy test? Did you read the directions and take the test when it was most accurate? Sometimes it is several days after your missed period.

Also, since you are on the pill (you have been taking it at the same time every single day, right?) you may have missed your period because of the pill. Sometimes your period gets really light where it's only spotting or you can not have it at all.

If you have been taking your pill every single day at the same time, I wouldn't worry too much. Talk to the doctor, just to be sure, but I think it's probably nothing ot worry about.