OPK Question...

So, I'm curious. I know that with opk you're supposed to test everyday around the same time, when the test line is the same/darker than the control you should ovulate 12-36 hours later. Etc.

I also know that once you get the LH Surge you can stop testing. But since my schedule has been all messed up - I took one everyday so far. AF is due in nine days - my question is after the surge I've read that it should go back to normal - no surge. Mine has been the same since my Surge on the 23rd. Just as dark every single day. What would cause that? Anyone had that happen? Results? Just curious why it wouldn't drop off as it has in the past...