Ovulation test amateur

Hi!
I hope I can find some women here who know a thing or two about ovulation tests
I’ve been testing for a few days now, but I’m not exactly sure how long my cycle is. The lines have been getting darker and darker, so I thought my ovulation couldn’t be far off. But today, the line suddenly became much lighter again. I test at the same time every day and never drink too much beforehand or anything like that. What am I doing wrong? Or is it normal for the line to suddenly become much lighter? I’ll add a photo, but I don’t have the rest of today’s results yet, so the bottom test is from yesterday.