OPK help!

Im on CD18 and haven’t ovulated yet. I just don’t understand what’s going on with my OPKs. They are light/negative in the mornings and dark/almost positive in the afternoons/evenings. I actually had a couple that were positive but ultrasounds confirmed they were false. I’ve only been using the digital opk with FMU as told by my dr and the instructions and it’s been completely negative every morning. I decided to use it last night too just for the heck of it and I finally got a blinking smiley. Has anyone had this happen?