opk test ?

So I started to test on cd 17 since my cycles are about 30-45 days . Anyway I got a very high almost peak test on the 17 but not positive yet so now I'm on cd 20 and it only keeps getting lighter and lighter after that first test on cd 17 , could it be that maybe I could of gotten a positive on the 16 and didn't catch it since I only started testing cd 17 and not 16? Any thoughts please.  ??