vaginapagina | Dollar Tree pregnancy test

So, my cycle is slightly late. Not unheard of, just not the usual. I'm at 38 days and my cycle is usually 30-35. I haven't had sex in 18 days. If I was pregnant would it show up on a test from Dollar Tree? I think they are 25mIU/mI of hCG. Or at least that's what I site I read said. I've taken three in the past three days. Each morning and all are negative.
I'm thinking my period is late because I had my paraguard taken out 30 days ago, which was the last day of my last period due to pain from it and heavy discharge. So, that may have screwed with my body. And the added stress I've had lately. My period feels like it wants to start, like the cramping, cravings, nausea and moodiness, but it just won't start. Why do pregnancy signs have to resemble your period so much? Ugh.
So, is 18 days too early to know?