HCG level

Hi! Could someone please explain how the hCG levels work? I had an ultrasound on Tuesday, and it showed I was only 5 weeks pregnant. I had another ultrasound on Friday because I didn't feel pregnant anymore, and it turned out the baby hadn't grown anymore. But could my hCG levels still be rising? The Clearblue test initially showed 1-2 weeks, and I took another test on Wednesday (so after the miscarriage), which showed 3+ weeks. Does anyone know more about this?