question???

If you go in for an ultrasound and you are suppose to be 6.4 weeks and they don't see anything, no sac, nothing. But then they say there is a heart beat. How does that make any sense???? Then they say you may be too early to SEE anything. WTH??? Then they say, see you in 3 weeks. 
No blood work done either. 
I wasn't there, asking for my son.