Condoms are a good thing to use.
But lets not dwell on that. You do know that the pill is 98.9 percent effective which means you could be pregnant. You might want to check.
Another option is that you just have a light period. Some times your uterus just doesn't have to much built up lining. So, I say just take a pregnancy test and if it turns out negative, you don't have anything to worry about. If you are worried anyways even after the test shows up negative, you can always go to a doctor/gynecologist and they can help you find out why your periods that way.
Hope this helped a little...