wiggle I'm 36, but feeling even more ancient after a week of cold. 
tomboy welcome to the thread. I do hope it turns out to be a nice proper positive BUT...
I am afraid I am another one who had a very unlucky experience post mc with retained tissue. (bear in mind though, that I think most of the people who didnt have classsic recovery, followed by quick pregnancy have gathered on this thread, so you will hear the worst of cases over here) Hope this doesnt sound too scary, but I had clear scans, AND later was found to have retained tissue. The only thing you can do at the moment is keep an eye on things, keep testing daily (get 25 cheap but accurate tests for a fiver on Amazon) and keep an eye out for any signs of infection (smelly discharge, abdominal pain, fever, chills) if you suspect infection go IMMEDIATELY to the doctors and demand antibiotics. That is really important because an untreated infection can cause scar tissue which can make you infertile.
This webpage has a good explanation of how rising and falling HCG can happen if you have small amounts of retained tissue. However, I agree with posts above that in your case it could just be that the doctors tests were not that sensitive, they may stick more strictly to reading the test within the time frame, whereas we all tend to stare at the thing for hours after. Do you know what your blood test level was? 30 is a clear pregnancy, less than 5 is not pregnant, so if it was at least single digits you are very nearly there in terms of getting your cycles back. You can ask for a copy of your notes at your GP's surgery anytime. They might make you pay for the photocopying though. The fact that nothing was seen at your scan is very good though, meaning that if there is anything there, it will be very small and quite possibly shed on its own with your period. If you can persuade your GP to do two blood tests 48 hours apart this would clearly confirm or rule out a pregnancy, see here for how it works.
Dach glad it looks like proper AF and that means you can get started sooner. thats great news.
enjoy the spa mouse 
stitch no news is good news 
Have a lovely weekend everyone xxx