Hcg Question

I am 10 weeks and 6 days pregnant, at 10 weeks and 3 days after having intercourse I started bleeding frank red blood,. Approximately enough to soak 1 sanitary pad. We went to ER, where there was a normal u/s and heartbeat. They did some blood work, including a serum HCG. which was 99,000. At no point did I have any pain.
The next day I saw an OB who sent me for serum hcg X 2 48hrs apart. I had another u/s that day and everything was normal, received an excellent report stating it was a viable fetus, baby was fine, heartrate was 172. The only thing negative was that my placenta was a little low.
The second HCG result was 95,000 and then the third one was 87, 000. I have a fetal doppler here and the baby's heartrate is still strong and above 150.
My question is that I have read that HCG can drop near the end of the 1st trimester and then level off eventually. Is this true? HOw much would the drops be?
Does it sound like I may be miscarrying? Would my HCG numbers have greater drops if I was?
The only pain I have is lower back pain, probably from all the bed rest and mild pain or more of an aching above my hips.