Based on my calculations, if your due date is 11/4/2010, then you conceived somewhere around February 11. If your "last period" was on 1/29/2010, then the conception date jives - meaning that you would have been ovulating around 2/11. I know you say that you only bled lightly for a day or two, but you also say that you periods often last only a few days. So...hard to say. Sounds like 1/29 could have been a period and conception on 2/11.
If you conceived on 1/24 or 1/25, then the ultrasound is off by about 16 or 17 days. That isn't impossible, but it is unlikely if you had an early ultrasound. Early ultrasounds tend to be quite accurate - within a few days usually. If you did conceive then, the light bleeding could have been implantation bleeding.
The only way to be 100% certain is to have a paternity test done, but I would think that the father is your ex partner who you got back with on 1/31.
Best of luck to you and your baby.
Well I am due on November 13th (but I have longer cycles). My last menstrual was Feb. 2nd and I ovulated about 17 days later so I got pregnant mid February. So you probably got pregnant about 2 weeks before me which would have been the beginning of February. If that is the case then I would say it's your ex partners baby but I can't be 100% sure since they were so close together. The only way to know 100% is to get a paternity test done on the baby. Sorry I can't give you a definate but it's a little too close. Good luck.