i fell pregnant whist changing pills, like from one brand to another, so it does happen.

as the other ladies have said it can happen while u are on the pill and it can happen as soon as you come off it

as for figuring out ur EDD id say get a ultrasound as i thought i was about 5 weeks pregnant when i found out and i was actually 10 weeks pregnant and wouldnt have known unless i had a dating scan to confrim. i was very confused about when i was due as i was on the pill and had no idea when my periods would have "normally" come IYKWIM