OPK help 👋🏼

Hey there! I’ve been tracking a couple of months and I’m confused. Lol

I had positive OPK one month on CD 17, one month CD 20 and this month maybe CD 18. I’ve just started taking pictures of OPK strips and I’m confused about why it started dark got lighter got towards a peak and then lighter. I definitely have the EWM the last two days.