What Causes Blood Clots With Positive HPT Test?

Okay I took a HPT about 3 weeks ago which came back positive and around the time that I was supposed to have my period I experienced very tiny blood clots which were dark red almost black in color. There was no period and I only saw them when I urinated. I also saw very dark blood when I wiped. The clots and blood lasted for two days. Could this be a miscarriage or implantation bleeding?