Late period, negative HPT.

So, I was using another app before I downloaded Glow for about 2 cycles. Both cycles, my period came 2 days before the app said it should. Well I downloaded Glow before this past cycle, and it winded up being I should start AF the same day that the other app said.. which was June 28th. So I assumed I'd start on June 26th. Well, on Sunday morning I was having really weird period cramps, so I went and checked and nothing there. I've been having these cramps on and off since then (along with being super emotional, super gassy, nauseous, no appetite, white/watery creamy CM, etc), yet AF still hasn't arrived. So I'm somewhere between 1-3 days late and I just tested this morning, with FMU, on a FRER, and it was definitely negative. So confused.. has anyone experienced this and been positive?