Help please!!!!

Hey ladies. I have a question and any help would be greatly appreciated. The first month I used Glow I tracked my AF and it lasted six days. I got AF on time in May right when Glow said AF would come she was there that morning. AF lasted only three days instead of six. Glow has my AF lasting six days instead of three days for May. How do I change this so Glow is right and my stuff doesn't get all messed up?