BFP last night, BFN this morning FMU??

Has this happened to anyone? TTC baby #1 for 9 months now and got a BFP at 7pm last night on clinical guard cheapie...this morning 6am BFN on cheapie and clearblue digital.....is there something wrong with testing with FMU early on? I am 5 days late and have been cramps on and off for a few days now. Any advice appreciated!