Help with glow

Lisa • I have a daughter who turned 4 in Nov 2015 I feel very lucky. I`d love a sibling for her, she`s also asking me for one x
So just joined glow a few weeks ago and it's been telling me I'm expecting my AF to show up on 6th December. Which was about right due to my calculations. My other app said I was due yesterday.
Anyway today I filled glow in on my previous Period dates. Now it's changed this months arrival date of AF to 11th December???
Which would make my cycle length like 40 odd days long???
I don't understand what's happened or how to fix it 

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