It's possible to get pregnant on the pill, but rare if you're taking it correctly. It's not possible to have a period when you're pregnant. Women do bleed when they're pregnant, but it shouldn't be akin to a menstrual flow. Your pregnancy symptoms might be side effects of the pill... How long have you been on it? If you haven't done so already, take a hpt, but I don't think you're pregnant if your flow is so heavy. You can make an appointment with your doc if you're concerned. :)