Like the others said, it can happen. I was getting BFN's when AF was due, and then for about 5 days afterwards I was getting an extremely faint line - even DH couldn't see it - he thought I was making it up! But then I finally got a BFP and now I have only 3 weeks to go till bubs arrives.

If you are able to be patient and wait a few days, that is the best way, but I was a serial POAS and tested every day!