I had a miscarriage 2 months ago please answer my ques.?

okay so, i had a miscarriage 2 months ago i was 10 weeks preg and had a d&c done. ive been going to follow ups to make sure nothing was left and the doctors said everything was cleared up. but then when i got my blood test results back my hcg levels did not go down. its been over a month and the hcg levels have not gone down. the doctor called and told me she thinks i am still pregnant and several nurses and people are telling me i could have maybe had twins and one survived. my question is if this is possible? that i did have twins and that one survived....the doctor is still unsure of what is going...