Positive then Negative?

So this morning I had very faint positives on 2 of the Internet cheapie tests. I wanted to confirm on a first response and figured since they are so sensitive it would show up. Well I used the same urine for the frer as I did the cheapies and it was negative! Any idea why? I know false positives are very rare so I feel like to get 2 is not very likely. Frustrating!