Bfn after 2 bfps (all using the same brand)

Hi,
Last Friday I got a bfp in the middle of the afternoon. It came up immediately and was as wide as the control line but not as dark. It was clearly visible to my husband and I. I did another test the next morning and it was positive too (same brand). Today (Monday) I went to the clinic and they did another urine test, the same brand as the others - they actually have me the ones I used at home - and it was negative! Not even a hint of a line. They did a quantitative blood test but I won't have the results until Friday at the earliest.
What could cause this to happen?
Thanks

Hi again,
Just in case anyone else has the same question, I thought I would update this thread with what I think is the answer: the tests were faulty. I've done 2 more tests using a different brand and they were both negative. I don't have the blood results yet but I fully expect it to be negative too.
The Better Health channel website (Pregnancy testing) says false positives could be caused by:
Detergents in the collection cup. I used new specimen jars for both bfps so I doubt this was an issue.
Reading the test after the time limit: both bfps came up immediately.
Evap line etc: both bfps were pink and the same width as the control line.
Chemical pregnancy: I've had chemicals before and always had a very heavy period within days of the bfp. It's now 6 days later and no AF.
So I really think the tests were faulty.
I had my implanon out three months ago. I had no periods for the 1.5 yrs it was in and haven't had one since taking it out. Think I'll stop wasting money on hpts and buy some opks instead.
If anyone has any other ideas I'd love to hear them. Thanks.

How upsetting for you. Although I wouldn't give up hope altogether until you get your results back. The only PG test I've ever used is First Response. Never had a problem with those.

Thanks. While the sensible 99% of my brain knows the bloods will be negative, I can't completely give up hope until the nurse says it to my face. I have always heard good things about First Response, might buy some and hide them from myself. I'm trying to walk a line between early detection for health reasons and becoming a crazy POAS addict. Definitely leaning heavily on the crazy side.