6w+1 and just a yolk sac on US

I went for a Ultrasound today bc I had slight spotting do they wanted to see me early. They saw the yolk sac but no baby :( the dr said she thinks I'm earlier than I thought- that I looked to be closer to 5 weeks. Thing is, my periods are very regular, 28 day cycles so I'd be surprised if I ovulated late. This happen to anyone? Where only yolk sac was visible at this stage??? I'm nervous this means I'm not going to stay pregnant.