BFP then BFN?

So I got my BFP this morning. I saved the FMU that I tested with to get the positive result and put another test in it about 30
Mins later and it said " not pregant." Then  tested three more times throughout the day.  I used One of the same CBD and it said "not pregnant" then two of the FRER  both had no second line not even a faint one. I AM SO CONFUSED!