the first day of my last menstrual period was december 17, 2006. i had sex december 31, january 1, and a few days into the first 2 weeks of january with my boyfriend with NO protection. on january 28 i had a one night stand with someone else (who withdrawed and didn't ejaculate in me) and 3 days later i took a pregnancy test that said i was already pregnant. on feburary 7 i went to the doctors for my first visit and they did an ultrasound that already showed the gestational sac and yolk sac. they said i was around 4-5 weeks and i just want to make sure if it is not possible that the one night stand guy is the father? i also had a 2nd ultrasound on feburary 27th saying i was 7-8 weeks and you could already see the image of a fetus. help?