Can i be further along than the doctors say?

trying to figure out when i concieved. i had sex two weeks before my period was due in feb. which was suppose to be the day of my ovulation and then took the plan b pill within the 72 hours as directed. then i had sex on the first day of my period which was feb. 25. my period lasted 6 days and was heavy and normal. i even had large amounts of blood clotsand it came on time. i then had sex two weeks later on march 12 which was the day i was suppose to be ovulating. my periods always come every28 days. my due date is dec.2. i need to know if it is possible that i concieved two weeks before my period or the first day of my period. i need expert answers. please help!