You should wait atleast 10 days for the HCG to be out of your system. I took a pregnancy test the 9th day after the shot and it was positive but day 10 and there on out was negative and got AF at exactly 28 days.
My nurse told me it takes 1 day per every 1000 units of HCG to get it out of your system, and the test wouldn't have shown you were pregnant that quick, the egg would have to implant and then you would eventually get an "HCG" in your blood. You should wait until your period is late or ask the doctor for a blood test.