Period late???

Gina
I'm kinda confused. I usually have a 23 day cycle so this app said I should have gotten my period the days ago. But then when it didn't come it change it til due tomorrow. No sign of period yet. Took clomid from Sept 8th to Sept 12th. When I go online it says im two days late but this app says its not due til tomorrow? Why did it change? And when should I test? I'm so nervous to get a negative that I'm waiting as long as I can, but I really want to know. Any comments would be greatly appreciated. Thanks