My husband and i have had sex after i had my daughter but i am on birth control but my period is very light. could i be pregnant?

Pregnancy unlikely: Pregnancy is not likely if using birth control correctly. One can do a urine pregnancy test, which if done correctly, gives a reliable result 3 weeks after sex (3 weeks after when conception might have occurred), or 1 week after a missed or strange period. If done sooner, a positive result is likely real, but a negative result may be due to testing too soon, and thus lead to a repeat test.