I didn't get a BFP until 40dpo (7w7d pg)... but then again, I didn't test before then either cos I'm a wuss and scared of the evil BFN monster.

I think B456 (Brooke) had a similar experience with her first, took a couple of weeks to get a BFP, I believe she was another 7-weeker.

Good luck, and if it keeps going then get a blood test, that will prove conclusively one way or the other.