Different tracking apps

So I've been using Glow and Flo to track my period. Flo says my period is 3 days away and glow says its 8 days away. I've been getting cramps. My lower back hurts, I'm out of breath. And I'm just plain tired. I just want to know if anyone else uses these two apps to track periods. (I started using flo with my period in September) Which app is more accurate?