I had a miscarriage 5 weeks ago. I never had a period. I went to the doctor today and my HCG levels are over 5000. I am pregnant again!!!!

Im excited but all confused today. I go in for an ultrasound Friday. My doctor wants to it. My question is, why would he want to do it that soon? Will he even be able to see anything? I highly dont think so. I cant be that far along if I miscarried only five weeks ago? Is there something they are looking for, but didnt tell me?