period question

Hey I was just wondering if when you've been following the glow app for your periods and it's normally right when it says your supposed to start it ... but then one period comes & glow is off? My period was set for the 12th and I've been pushing it back because it hasn't came, I usually don't get my periods until the 20th but last month I got it the 17th & was off by the 20th that's why I thought glow prediction was correct. I'm just wondering is glow wrong or am I late? Also I had heavy heavy cramps the day glow said I was due to start