Hcg still rising after a miscarriage

Got told I had a miscarriage I started bleeding and passing clots Wednesday no cramps just back pain. Bleeding stopped Thursday and had spotting friday and nothing since now monday. Hcg last monday was 813 then 809 Wednesday and 827 following Monday. Can they still rise after a miscarriage? I had a abdominal ultrasound today and they told me it was a complete miscarriage. Gng from my period dates I would of been 8 weeks. Im so confused. Back in Wednesday for more bloods and then if there still rising a vaginal scan.