Fluctuating OPK?

This is my first month of using ovulation tests. I started as my period was ending, about 6 days ago. There hasn't been a positive one yet but the darkness of the test line fluctuate. They started quite visible, but not positive. Then they have gotten lighter... One day will be darker, then nothing at all. I'm even trying twice a day. I'm new so I don't know a whole lot... Has anyone else seen experienced it?!