Using word via onedrive. Problems with page lengths changing

I'm writing a book with the onedrive word version. Whenever I type, the page bottom/top will disappear and be replace with a dotted line, and I'd be fine with this except that while doing this the page height will expand or contract, messing up with the paragraphs. It's gotten to the point that I've had to stop writing completely. Can someone tell me what this is and how to fix it?