If you had a period after the guy on October 20, the dad is not the guy from October 20. Not only did you have a period (a pretty much slam dunk indicator that a woman is not pregnant) but also, if you had the period begin on the day after sex, you would not have been ovulating at that time. Your ovulation would have been more like on the 7th of October. There would have been no egg in your system waiting for a sperm on October 20. Sounds like you got pregnant on November 4.
Plugging your information into a pregnancy calculator, it gives you a pretty wide range of possible conception dates. Their estimation is the pregnancy resulted in sex that occurred between Oct. 27 and Nov. 11. Best wishes!
What was the date you started your period, and what due date did they give you when you had your first ultrasound, when the sac was measuring 4.1 mm? Also, what was the date of that first ultrasound?