Hi
You may be describing the result of accidental watch face interactions.
Under the current version of watchOS, swiping from edge to edge across the watch face switches between different watch faces that have been added to the collection on your Apple Watch.
If you tend to only use one watch face, you may find it most convenient to remove any others from the collection on your Apple Watch, so that accidental swipes across the screen no longer select a different one.
Watch faces can be added back as and when desired, either directly on your Apple Watch or via the Watch app on your iPhone:
The following steps may otherwise help (check after each):
If you have not yet done so, update your iPhone and Apple Watch to the latest compatible software versions, starting with your iPhone:
- Update the iOS on your iPhone, iPad, or iPod touch - Apple Support
- Update your Apple Watch - Apple Support
Close the Watch app (if open) on your iPhone and restart both devices (normally):
- On your iPhone, double-click the Home button, then swipe up on the Watch app preview.
- Restart your iPhone, iPad, or iPod touch - Apple Support
- Restart your Apple Watch - Apple Support
Unpair and re-pair your Apple Watch:
- The Watch app on your iPhone takes a backup of your watch automatically when unpairing via the app.
- Initially choose to Restore from Backup during set up (most data and settings will be restored, with some exceptions, such as Apple Pay cards, passcode).
- If the issue persists, consider instead unpairing and setting it up as a new watch.
- Unpair your Apple Watch and iPhone - Apple Support
- Set up your Apple Watch - Apple Support