I started my birth control pill about three days ago. i had sex last night and according to my app i start my fertile window tomorrow. could i be preg?

Hard to guess: One can do a home urine pregnancy test if worried. A urine pregnancy test, done correctly, gives a reliable result 3 weeks after sex (3 weeks after when conception might have occurred), or 1 week after missing a 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.