No one, not even your doctor, can tell you exactly when you conceived. It's not immediate after sex. Sperm can survive up to about 5 days or so as well.
With a period starting on January 31 you'd normally ovulate between Feb 11 and Feb 16. The egg is viable for about 48 hours.
Your due date does not indicate when you got pregnant. It's based upon your LMP. Why? Because most women know exactly when that was. November 7 is 40 weeks after January 31.
You need a DNA test. Unfortunately there is no other way to know who the father is. It can be done now, as early as 9 weeks after you conceived. Look up "non-invasive dna testing." It isn't cheap. They obtain the babies DNA from your blood. Both of your partners would also need to donate.
Wish I had better news for you.