Testing out my trigger question

So, I've been taking pregnancy tests so I know about when my trigger shot leaves so I'll know for sure if I do happen to get a BFP from my IUI.. And today the line got darker? I guess my question would be, why does this happen? It's too early for me to tell if I'm pregnant and I know that and accept it, but I was just curious why the line would be darker than the previous days. It was getting lighter but now it's significantly darker and I'm just wondering if any other ladies have had this happen to them?