first of all, birth control isnt guarenteed, as with condoms. birth control also doesn't prevent STD's. you need to wear a condom no matter what, they have many kinds(regular, diaphram, female condom, etc). also when you use some kind of female condom, i think a diaphram, there is this foam you put on its spermacide which helps prevent std's and pregnancy so thats good, and you can condoms with spermacide...im sure you know this all though. but please, use a condom. im not sure if you could be pregnant, but if your nautious and have headaches it sounds like its possible. go see your ob/gyn.
If you don't take the pill correctly, then that's just like not taking it at all. You can become pregnant doing so. However...since no semen actually came into contact with/inside the vagina chances of becoming pregnant are pretty slim (but anything's possible). The period to me just sounds like an after effect of missing pills...it messes up your cycle quite a lot and confuses your body when you miss pills, hence the heavy bleeding. The nausea etc could be from a number of things.