I started a medium dose 21 birth control pack a couple days before my period, and switched to low dose, but my period hasn't stopped for 11 days!?

A new pill regimen: You switched pills and went from a higher to a lower dose so you may be having "withdrawal" bleeding. Always check for pregnancy if this is your first time with abnormal bleeding while on pill. If not pregnant then continue with your pill of choice, be patient and if bleeding thru middle of pill pack does not resolve after 2-3 months, then switch the pill. See a gyn.

Bleeding from pill: It is common to have irregular or unusual period bleeding when first starting the birth control pill. And it's more likely if you change pill types or start the pill just before your period. Keep taking what you are on and it should stop soon. If it doesn't check with your provider.