Someone Please Help!

Okay here goes. In december of 06', my partner of 7 years, we decided to take a break. Well on december 20th, I had sexual relations with another man and on the dec. 22nd I took the morning after pill. Two weeks later, on jan. 6, I started my period, it was lighter then normal and only lasted two days. But it was a constant flow. Well february of 07, I didn't get my period. And on feb. 26th, I went to the doctor and we had an ultrasound, which dated me at 7 weeks (gestational). My partner and I got back together on the day before christmas and have had intercourse often since then. All of the ultrasounds that I have had, have been consistant with my last menstrual. But there is still doubt in my mind. Is there any way it could be the other mans. I only had intercouse with him that one time