Do you have pictures of your tests? I haven't done IUI, but you get a trigger shot, right? When is that supposed to be out of your system?
Unfortunately I have had two early losses where I had nice positive tests, even progressing the first 7 days, then fading away after that.
No one can really tell you if today's test was faulty. Of course there are a lot of variables- concentration of urine, how much dye is in a particular test (even of the same brand), etc. But if it's much lighter than the previous ones, it might not be great news.
I hope it was just a dilution issue or something. If I were you, I'd take another tomorrow and see how it compares. Good luck. <3