+OPK changed expected period date

This is my first month TTC and I had a +opk on CD15 (yesterday) with a 26 day cycle. When I logged the +opk the app moved my expected period date back by 3 days. Is this normal? Do you think I should still test on the day AF was originally expected, if I don't get it? Or should I wait until the new date approaches? The 2ww is killing me and I'm not sure that a +opk would change the date AF is expected. Any and all input would be greatly appreciated. Thank you!