Baby heartbeat


Ok so the first date of my last period was 28th of september...two weeks later started getting real bad cramps to the point i couldnt sleep...1st nov got a bfp and went to doc who said might be ectopic so got a scan that showed a sac but no heartbeat and put me at 5 weeks 4 days....went to doc today said i should be 10 weeks???? Wtf...asked me to do blood test as i could have a missed abortion....can anyone tell me what's goin on ?...when can they see heartbeat....still have the cramps