Yeah, I was having some spotting too. But they said my hcg levels were ok. Just my progesterone was low. I am also taking the suppositories twice a day (and I leak for several hours after I take them... it's a bit annoying, but, of course, worth it). I am going to have a sonogram at about 8 weeks (May 31).
I know you will be worrying anyway, but I would think the heartbeat is a great sign. You don't have to worry about a blighted ovum anyway, and the progesterone should help keep it in place.