Please help HCG

I got my blood drawn and my hcg level was 11000 and then 48 hours later got it drawn again and it was at 18000 one doctor called and said it was nothing to be concerned about and scheduled me an ultrasound but now i just got a call from another doctor at the same office saying "most likely" i will miscarry. Wtf. It got me angry because yes it didn't double but i thought after 10000 it takes longer. She said they weren't 100% positive but "most likely"... Has this happened to anyone and they did have a miscarriage? Or had a healthy pregnancy? Should i be worrying? PLEASE RESPOND!