If you are switching birth control pills and switch after your period week, do you have to use a back up birth control method?

No: If you are switching from one brand of BCP to another brand and starting on the day you would normally start a new package, you should have extended coverage. Also, if you have not been on the pill, but start using BC pills within 3-4 days after the onset of menses, that should also be effective without needing back up birth control. A reminder: BCP is not adequate for infection prevention.