Claire - Sorry for the TMI question, but did you have red bleeding when you thought you had AF? If it was pink/brown, even if it was for several days, it might not have been AF.
On the other hand, I have heard a lot of bad things about the one step tests. Wondfo brand is much more reliable (also cheapies on Amazon) but most of the name brand tests ant a pharmacy will be better (albeit more expensive).
Good luck!