Home pregnancy test are pretty dawn good. Trust a good middle of the road, price wise. They can pick up false positive and false negatives, with anything there is error. I don't thing it is a cross with the yeast, because the test works by binding to particular hormone chemical structures, very unique to that hormone being tested. Always differ to blood test if in doubt. Sorry I don't have more of an answer.
HOPE THIS HELPS
I've taken pregnancy tests before and gotten Negative, but then looked back a while later and I could swear there was a faint line - only the packaging I think says not to interpret results more than an amount of minutes. I think since you double checked with another test and it confirmed another negative, that's it proabably not a BFP. That's my opinion.... :(
I know it's so disappointing! I do the same thing with my OPK tests!
Good luck and sticky baby dust to you!