If you had a mc the urine tests would still show positive for a short while afterwards. Only 2 blood tests to see if HCG levels are rising or falling are an accurate way to tell (other than just waiting and testing again in a month, but I know how agonisingly impossible that wait would be).

Have you bled any more? I am wondering if the sonographer grazed you when doing the ultrasound, or perhaps knocked your cervix. Do you ever bleed after sex (often caused by cervical erosion which basically means the outside of the cervix os more like the inside and often bleeds if knocked).