OPK Confusion

So this is my first month trying OPKs. I started taking them at CD6 just out of curiosity. Since then I have taken them every few days to try to get an idea if I ovulate earlier than what glow predicts, etc. My estimated fertile week is coming up starting tomorrow, and I took a test tonight and to be honest my line was darker on CD6 than CD21 (which I’m on now). There practically wasn’t even a line tonight, so I don’t know if it was invalid? (I’m using the First Response kit).

Also this morning I had some weird very very light brown discharge that lasted for an hour and then went away, but was followed by some mild cramping and a backache . I’m not sure if my hubby and I have just been BDing to rough lately? Anyone have any similar experiences or thoughts? Thanks! ❤️