And by saying that you bled for three days on the 28th of "this month," you mean January?
If you got pregnant in January, given the regularity of your cycles, everything would suggest the one-night stand guy and not your ex, as the dad. Please get an ultrasound next week if at all possible, or one week after that, and ask them to give you an estimated due date based only on the measurements of the embryo and its developmental markers (not counting in the date of your last period). Using that estimated due date, you would be able to figure back to an estimated ovulation date either by counting back 266 days on a calender or using a conception calculator online. And remember that when the doctor says you are x weeks pregnant, he means since the first day of your last period, not since conception. So counting from December 28, you are presently 6 weeks 4 days 'pregnant' but only about 4 weeks 4 days since conception.
I'm usually right on time. 28 days
How regular are your cycles, and how long (meaning, from day 1 of bleeding to day 1 of bleeding the next time)?