Font size changes when on email. It becomes tiny. Anyone experience this?

My customer has an iphone6. We use Lotus Notes Mail and use the iPhone's native mail app. Her phone is on OS 8.1.3

The body of the message shows tiny fonts but the header is on a regular font. It was fine before but changed suddenly. She restarted her phone and it's now back to normal. What could have caused this?

Thanks!