Help... I had a 1 night Stand March 13 and had with my boyfrend to find out I am 7weeks and 3 days preggo!?

I had a 1 night stand protected and the condom slipped inside me on March 13th ... I have with my boyfriend numerous times after this ( March20th.. March 28th and twice in April) and on May 24... I had a ultrasound that dated me 7 weeks and 3 days pregnant... Which says that I became pregnant around April 2nd... I know it says a marginal error of 3 to 5 days depends on the fetus growth... am i pregnant by my boyfriend or the 1 night stand... are the ultrasounds accurate in 1st trimester... HELP PLEASE...