I'm getting really confused...

I thought you could get pregnant at any time; that there's always a chance.. Heck, I heard that you could even get pregnant before you start having your period... Don't remember where I heard that one, though...

Also, I thought that when you were on birth control you weren't really having a period either... That the pills made your body think it was pregnant, and then confuse it or trick it into shedding the lining...

Is any one else kind of amazed at how many different theories/thoughts there are out there about something as common as birth control?