Late but no positive test...anyone else?

I typically get period first few days of month. Im NEVER late! Just started us I my this app a month ago. Said I was going to get af the 8th. Never got it. Then the 10th. Still no af. I've had mild cramping but nothing crazy. I also had cramping about 2.5 weeks ago. I've had mild backache too....anyone have any thoughts or been through this or even have an idea on why I'm late?