IUI, Hcg shot, BFP

I need help!!! So I got 10,000 unit hcg shot on Friday may 22, felt ovulation that night at 9:30, and had our second month of IUI on Saturday morning. Last month I metabolized the hcg shot pretty quickly and it was out of my system by 7 days after the shot. This cycle, I took a pregnancy test Wednesday (5 or 6 days past the shot) and got a line SOOO faint that I had to ask a coworker (I work at an obgyn office) if she could see it. Today, 9 days past the shot, I got this faint positive. Is it possible for the hcg shot to have held on and still be in my system, causing a false positive? Could it be that they are different brands? (Both brands used detect 25). Opinions?! I'm going to take another test tomorrow to see if it gets darker. I just don't wanna get my hopes up if it's a false positive.