Okay. So, according to the doctor I found out I was 5 weeks pregnant October 3rd. On October 26th, I was 8 weeks according to my ultrasound and my LMP. So, could I have gotten pregnant September 21st-22nd, or would I have had to been pregnant before then?

You will have to have a DNA test done to determine paternity. Estimating gestational dates is nothing more than an educated guess.