Well, you calculate how far along you are by the first day of your last menstrual cycle. So, that would have been January 25th...as long as you didn't BD with Bryce after that, then its pretty safe to say that its Jamie's. But, Suzie is right...going to the doc and getting an u/s will help you verify how far you are. But it sounds to me like you got preggo when you BD'd in the beginning of February...ovulation usually happens approximately 14 days after the start of your menstrual cycle...goodluck!
Have you had an "official' ultrasound yet? Having an u/s done will give you a much better view of your dates. Keep me posted