I'm Pregnant My Due Date Is July 3rd Which Was

I'm pregnant my due date is july 3rd which was given from obgyn, I went to the hospital on november 16th 2015 they did my first ultrasound, they were able to mesure the baby and see a heartbeat but they gave me a due date of july 9th. I had sex with my ex boyfriend once on october 24th 2015 could he be the father, or was I already pregnant