iPhone 7 Plus - Mail problems

I have the iPhone 7 Plus, iOS 11.1.2. I have been having problems with the general Mail app, but it started on my old iPhone 6. At seemingly random times, I will open the Mail app and all of my emails will have been wiped out and are regenerating and downloading. Since my email is Yahoo, I have since downloaded the Yahoo Mail app to use. However, I am still curious about why it will randomly clear and regenerate my mailbox. Anyone else have this issue or a solution? Thanks!