OPK question

So today is CD 11 and I normally have 28 day cycles (+/- couple days). I started testing twice a day 3 days ago, and the lines have all been pretty light but getting darker. I tested today at 1pm and it looks lighter than the one from last night.. I'm so confused! I'm thinking I might have missed my surge even though I test twice a day, but it's still early in my cycle and I'm not supposed to ovulate for a couple of days still.
Can anyone shed any light on this please? Have you ever had your opk get lighter just to get a positive a couple days later? 
Thanks everyone