I Believe I Am 8 Weeks Pregnant But Had An

i believe i am 8 weeks pregnant but had an ultrasound yesterday and only measured 6 weeks, no heart beat only a sac. the dr's keep saying that my dates are wrong but i know my last period was july 29, the first day of my last period and i was ovulating aug 12 which is the only day we had sex during that cycle is it possible to only be 6 weeks or am i having a false pregancy