Could sex on Sept 20th have gotten me pregnant when my due date was ultrasound dated to be July 11.…

Could sex on Sept 20th have gotten me pregnant when my due date was ultrasound dated to be July 11. What would my due date be if I got pregnant from sept 20th considering the sperm could have hung around and then impregnated me. I had bleeding in the begging of oct but I fear it was implantation bleeding from sept sex