I'll chime in. I got my bfp on oct 31st. I was using ovia app to track my cycles. They had me as fertile from the 17-21 On the 18th I picked up an opk because I was curious if I could get a better window. I test low on the 18th, 19th, and the 20th. I started getting high results on the 21st when I am assuming I implanted around then. Because if all is correct then I ovulated on 18th which would explain why I got a low because I missed my surge. I had highs until I ran out of strips.