All OPK users.....I'm so confused!!?

I've been ttc for 3 months now (for our 3rd). First two pregnancies I got pregnant within the 3rd month of trying. Anyway,last month was the first time Ive used a opk kit(the dip stick kind).The confusing thing is when I hit the 18th day of my cycle, I get a positive.......but just half of the test line is as dark as the control line. I also keep getting positives for up to 4 days later. What gives? This month I purchased a different brand (Target) just to make sure the first one( Answer) wasn"t the problem. So like clock work. 18th day comes around same thing happens.....half a dark test line and for 4 days.....This is driving me nuts...Can anyone with opk experience help me out? Is this normal? Thank you so much