Hi Jo,
I TOTALLY know what you are going through at the moment.
I have two stories to tell you.

With my first pg I had some spotting at 5 weeks and went to have an u/s which showed nothing. Too early they said. I had my hcg levels monitered every 48 hours. My levels were just in the normal range but on the low side. They were taking five days to double. My doc wasn't very hopeful and neither was I.
The spotting continued and I had further scans to check for ectopic pg. This was ruled out at 6 and a half weeks when they saw an empty gest. sac in my uterus. Still too early to tell what was going on they said.
The spotting got heavier until I was 7 and a half weeks and they got me to a specialist. I had a scan which showed the sac was too small for my dates and still no fetal pole. I knew my dates couldn't be more than 2-3 days out so I knew then that it was a blighted ovum pregnancy.
At 8 weeks we got a definate diagnosis and I had a d&c a few days later. My hcg the day before my d&c was 30,000 so it was still rising slowly.

Then after three months TTC again I got a BFP!
My doc assured me that a blighted ovum would not occur again.
At 5 weeks I had some spotting and of course assumed the worst.
I had my hcg levels checked and sure enough they were taking five days to double again. Oh no. The level was higher this time but the doubling time seems to be the indicator of a viable pg.
I was prepared for the same scenario as last time but I went for a scan at 6 weeks 3 days and we saw a baby and a heartbeat. The specialist has very sensitive equipment so was able to confirm a viable pg very early. The hormone levels are a worry but they obviously hadn't had a negative effect up to that point. The spotting has continued on and off but the ob. was able to see where that was coming from, nothing to do with the baby.
I am currently 8 weeks 1 day and so far so good. I haven't been having my hcg monitored any further as the ob said it doesn't mean as much as the gp's think and he didn't want me worrying.
I am off for a scan on Monday and hopefully all will be well. We are not out of the woods yet but at least we have a good chance.

I really hope that I haven't scared you here, I just want you to know that the hcg isn't really as reliable as we are led to believe. Another scan will tell you what is going on. I am thinking of you and [-o< that things work out for you.
If you want to know more just ask.