My ultrasound says i'm 5 weeks 6 days. mar 11th i took morning after pill, bled lightly. mar 16th had sex, mar 25th heavy period. when did i conceive?

Go by ultrasound: Early ultrasounds are pretty accurate in determining how pregnant you are, and estimating when you conceived. Given scenario you presented, it may be hard to determine when you conceived based on dates you presented. If you had a heavy period on 3/25 that lasted several days, then that was likely your Last Menstrual Period. However, that is mute now that you have a gestational age by ultrasound