Opks with lines

Abbie
So the opks using the lines, I've been taking them since CD7 and CD15 I got a positive or a very nearly positive and I'm now CD18 and I still think it's positive? It's so hard to tell as I find them difficult to interpret if the test line is actually darker but I'm pretty sure they are on all of them. Is this normal? Does it mean I'm yet to actually ovulate? 
Thanks!!