Confused!

I took an ovulation test on Thursday, solid circle, took one yesterday morning and it was a smiley and the smiley is still there. However, I was curious and just took one and it came back with a solid circle. How is that possible? Could it be that I took the test too late today and not first thing in the AM? I thought when you got the first smiley that was the first day of your fertile window..is that wrong? Has anyone experienced this before? Any insight welcomed! Thanks :)