So glow was wrong about my ovulation?

I know this app isnt set in stone as far as reliability (esp if you have really irregular periods) but I got AF yesterday (sad about that) and because I started 2 days earlier than was predicted, it moved my ovulation back 2 days so I basically had sex on my O day if glow is on track in any way. I'm sad that Im not pregnant but I also know the POM is not a neither birth control nor is it much possible to conceive from it. We weren't trying but we aren't preventing.