I'm so... confused....

so this morning I started HCG shots for a weighht loss program I am doing. now they told me it takes 72 hours go get in my system and that I will show positive on a pregnancy test after. I meant to test before I did my first HCG shot this morning because I am 4 days late for me period. but I was running late so I did the shot and took a pregnancy test about 4 hours after the shot and I got a big positive. I was so shocked that my mother who started the HCG shots today as well I had take a pregnancy test and hers was negative. I don't want to get to excited but I can't see that I would be late on my period and get a positive test if it was to blame on the shot that I did for the first time today. does anyone have any experience with HCG shots?