Can you help?

Why is it one day your OPK strip would be negative but darker and look as if it will get to a positive soon. Then the next 3-5 hours test again and it's back to lighter and more negative than prior? Like how does it go from darker to lighter but never peeking? 
Does this happen to anyone else it drives me crazy! Especially when I haven't ovulated yet and it's getting almost past my fertile window.