AF came early!

So this is my first month properly tracking ovulation using Glow. AF should be due in 4 days. I was on the pill for 9 years and AF ran like clockwork, arriving every 4th Friday. This continued when I came off the pill and has been the same for the 9 months since. Until this month. I started to use Glow and she came early. Just as I thought I was getting to the end of the TWW. She's never early. I'm now thinking it will disrupt Glow calendar. Think I better buy OPKs next month to be sure. Anyone experienced this?