question about conception date

I'm a little confused about my conception date. For the first time I made a mistake and slept with two different men in one month. One was my ex husband and the other was a new guy that I thought was right for me, but ended up not being so. I had unprotected sex with my ex husband on the 11th. Which I know for darn she he "came" inside of me. I met the other guy on the 15th. We did NOT have sex on that day, But we had protected sex on the 16th. Unfortunatley I was confused and had sex with my ex husband, which was unprotected, the night of the 16th. Again, I had sex with the other guy on the 18th, which was also protected. My ultrasound on April 24th was 7 weeks and 2 days. My due date is December 8th. When I got a positive pregnancy test on the 4th of April, I was 4 weeks and 5 days. My LMP was 03/03/08. When I count back 7 weeks and 2 days from the 24th of April, It lands on the 11th. The only time the other guy and I had unprotected sex was on the 29th of March and my expected period was supposed to come on the 30th of March. I'm very nervous, because my ex husband and I are trying to work things out and he's extremely happy about the baby. I've never been in this situation before, and I'm kicking myself everyday because it seems that I can't figure this one out. I've been told that there is a possibility that women can get pregnant while using a condom if it's not used properly, but I know for sure this condom was used properly. I know for sure that if this isn't my ex husbands baby, we will never be able to work things out. That's just the way he is, and I completely understand. Could someone please help me? I'd greatly appreciate it. Thank you. =]