My girlfriend and I have never had intercourse, but we do like to cuddle and while she rolled on top of me her underwear slid a bit and made contact with my somewhat exposed erected penis ( I was wearing underwear, but part of it was revealed from rolling around) that may have had pre-cum. She didn't tell me right away, and I ended up fingering her (my fingers were clean). I was told that my penis made contact to the side of her vagina, and I began to worry about somehow rubbing pre-cum around. I told her to shower the side right away with soap in case there was any soldiers taking refuge. She is also 4 days into her period.
My expierence with birth control pills is if you miss one day I would always start bleeding the next day. But I also know that when you stop taking birth control a woman should be able to get pregnant the following month. Sperm does live in the female body for 5 days, but if your bleeding during intercourse then the blood should push the sperm out along with the blood. But anything is possible and woman do get pregnant on their periods. So if you are not trying to have baby use alternate protection. Birth control only prevents so much.