When did I conceive?

Ok so this gets complicated. I had sex with my SO before he went out of town in Mid September. After that I had a light period (implantation bleeding?) about 2 weeks later. I decided after that to get off bc to try to conceive when my SO got back. We had sex when he got back in Mid October. Right after Thanksgiving I found out I am pregnant. Ultrasounds have all measured as conceiving when my SO was out of town which isn't possible. 3 weeks after he left or 1 week before he got back. Help?