Fertile window.

Ta
I input all of my data into glow. But I calculate that my AF should arrive on the 31/03 but glow is saying that it should be 28/03. So I don't really know when my fertile window is? Because glow is saying that it would of been last week. But if my AF is due 31/03 because normally roughly it's 33 day cycle. Should it be this week? Confused. Please help. Thank you.