Please help me understand.

So I went to the doctor yesterday just to find out I am pregnant. My period is now 6 days late but the doctors are saying I’m about 4-5 weeks now but why are they counting when I was on my cycle and not from the approximate time I ovulated. Also, they only seen the sac and not the baby!!! They said I may be so early that we can’t see it yet but at 5 weeks you should be able to hear the heart beat! I’m soooo confused, scared, nervous, pissed, happy, excited, overwhelmed, tired, and more scared. But I think that’s just part of it all lol. Please help. Somebody anybody!! 😞