Heartbeat but no yolk sac

Jackie
Just got back from my doctor appointment. Due to irregular periods I don't know how far along I am but I would guess 6 weeks...? We heard a strong heartbeat (135) but they didn't  see a yolk sac. Anyone know anything about this? What does that mean? The doctor didn't say much except that it's very early and we will just have to wait and see what the next scan shows in 2 weeks. Has anyone else seen/heard this with their scan?