The face changes on its own?
The face on my Apple watch arbitrarily changes , without my input?
It probably happens when you inadvertently swipe the watch face. See the 'How to switch the watch face on your Apple Watch' section in:
Change the watch face on your Apple Watch - Apple Support
Perhaps your shirt sleeve is making contact with the watch face. You may be wearing you watch too loosely and inadvertently pressing the side buttons or swiping the watch face:
Wearing your Apple Watch - Apple Support