My "smart"phone has started doing something weird today and I can't understand why

I was trying to open and reply to emails on my phone and it's started literally shutting gmail down when I try opening emails, will literally close it?

I don't understand why it's randomly started doing that?

I had to wait until I got home before responding to emails

I've always had problems with it when I've had more than one email coming through together, opening a email causes it to crash - happens every time

but this whole gmail closing itself down thing on my phone is something new ?