Almost positive OPK, now fading- help!

This is our first month ttc, and I've never used OPKs so I started testing just a couple days off my period, so I wouldn't miss O. The line was pretty dark, but possibly not as dark as the test line. Every test since then, the line had gotten lighter and lighter. But I'm just now beginning to get slippery mucus so what gives? I'm just now nearing what most calculations say my fertile window should be so why the weird opk results?