Menstruation question?

My girlfriend has been on the pill for about 5 months now and has yet to have her period on the "sugar" pill week. We recently were having sex without a condom and I was getting ready to climax and she wouldn't lift up and I went inside her. She kept going until she climaxed. This was a few weeks ago and now her period is suppose to be this week but instead of a full fledged period she is just spotting. Is this something to be worried about? Or is it just a reaction to the birth control because her period seems to still be adjusting to it.