Lighter period?

How likely is it to get pregnant on the pill? me and my boyfriend never have unprotected sex but there was one time about 2 weeks ago that he put it in for about less than a minute but didn’t finish inside.. i’m always good about taking the pill i take it at the same time if not maybe 10 minutes later then normal every day. but this period i got sunday was not heavy at all, i’ve had a really bad headache since then but no cramps or really any bleeding? normally they are heavy with bad cramps. i’ve been on the pill for just over a year. is this normal?? to have usually heavy periods and then one not really any blood period?