help!!!! still new to opks

Heather

I took the opk in the bottom photo on the 2nd of may and it was positive. I actually took two tests, both very positive. We did the babydance and the next day when I tested the line had faded and was negative. Each day the line got fainter until today. The top photo was taken today. Why did it get so dark? I should be 6 days post ovulation.