Rising hcg

Doctors have told me I have had a missed miscarriage at 6 weeks (should have been more like 8) because there was no heartbeat. I have just recieved my results for hcg levels and they are still rising but aren't doubling so they are saying it is still a miscarriage. I haven't had any bleeding or spotting, no cramps and still have all the pregmancy symptoms. I thought the levels would have been dropping?