What Could Cause On And Off Spotting And Bleeding, HPT Showing Strong Positive After Surgical Abortion?

I had a surgical abortion 5 weeks ago. I was 7wks 2dys pregnant. Since then I have been spotting and bleeding on and off. I went back to the clinic where they performed an external and internal ultrasound and said there were no retained products. I have not had my period yet and continue spotting and bleeding on and off. Yesterday I did a home pregnancy test in the afternoon which showed a strong positive result immediately. When I had my blood tests at 5wks pregnant, my HCG level was 10,500. Could it be leftover HCG or could this be a new pregnancy? I would have thought the HCG level would be back to normal, or perhaps I'd get a faint positive after testing in the morning? But as I said, I tested in the afternoon and it was an instant strong positive result! What is going on?