I'm working in pages and it won't let me save my document. A popup appears saying document could not be autosaved?- Why

I am working on a document on Pages on my Macbook Pro (High Sierra) and keep getting the following warning : The document “Untitled 18” could not be autosaved. The file has been changed by another application.

I know I can override it but it happens with every doccument. What is going on?

There is no other application that could be changing my document and although I have an iphone, there is no link between Pages and my phone that I am aware of. Help please, it is highly annoying.