I understand why you're upset honey. I just want to add a little more information. If you did lose a twin which it would seem is almost certain... Then this will fiddle with your hcg results. So, there would still have been residual HCG from the other embryo. This could look like less of a rise than really did occur for the baby that is there.

After a heartbeat is seen on u/s the best indicator of wellbeing is the sight of a consistent and adequate heart beat & of course growth. You have this so that is an incredibly positive sign.

So - whilst that rise could seem deflating I have personally seen this occur from a twin pregnancy where one has flown away. I hope & pray that your baby stays...