period just started so i started my birth control...

Likely a bit shorter than your normal menstruation as the pill will stop your uterus from shedding its lining as the concentration of the hormones increase.

its been 7 days! why am i still bleeding

It is likely breakthrough bleeding, a side effect of the pill during the first few cycles. Give it a few days and speak to your doctor if it does not stop, or it is very heavy.