Conception dates, Pregnancy Tests

I am about 25 weeks pregnant. i have a consistent 30 day cycle. My last period was February 19th, 2012.  I had sex with my boyfriend all month including March 7th.  I had sex with another man on March 10th. I tested positive for pregnancy march 16th using First Response home test. I have a daughter and lost two infant sons with my boyfriend in the past. This baby is a boy and he is measuring smaller and in a much lower percentil then the other 3 babies I had with him.  I am very worried this isn't his baby. I have three questions. Are the ultrasounds ad indication of a different father? Do new, modern, more sensitive pregnancy tests make it more likely I could have tested positive just 6 days after possible ovualtion? Is a ten day luteal phase unlikely?