Could he be the father?

Ok rephrase.. I had on the 27th n I took a preg test on the 3 which tested positive. (Lightly). I went to the doc on the 14th and she performed an ultrasound which put me at 4wks. Could he be the father? it had only been 6 days. And my ultrasound put me at 4wks. I don't have periods.