Docs say I got preg mid aug, DD is May 10/15, only sex in aug was aug16/14 first u/s was 5w 4d on sep11/14, I was able 2 see heartbeat, this possible?

The rule is... If it happened the way it happened, then it is possible. There would be no other plausible explanation. So, if the only time a woman had sex was in the middle of August, that would be the only time she could have gotten pregnant. If a fetal heartbeat was seen on a prenatal ultrasound scan, then it is possible, regardless of whether or not it was surprising to anyone.