What am I missing? (& August sucks)

So I got my mirena our last month after 4.5 years and AF came Aug 2-4 (super light and short period for me, which I attribute to the IUD). I use my clear blue OV kit and it says I’m at peak ovulation on 17th. So we got down to business. On 8/27 I take a pregnancy test (BFN) - which is fine, I expected it. But then fast forward to today (8/29) and AF is back again already?!? So like, was the ovulation kit and glow prediction wrong? Was i or was i not peaking on 17th? Note, today doesn’t seem like typical “just spotting.” This is AF. So yea, I’m very confused (and new to this - my 1st was unplanned). Help!