Q: Apple Watch screen flashes instantly

Hi

That behaviour is not normal. The following steps may help (check after each):

• Restart both your iPhone and your watch, turning both devices off together, then restarting your iPhone first:
  • Restart your iPhone, iPad, or iPod touch - Apple Support
  • Restart your Apple Watch - Apple Support

• Unpair and re-pair your watch:
  • The Watch app on your iPhone takes a backup of your watch when unpairing via the app. Choose to Restore from Backup when offered the option during set up. Most data and settings will be restored, with some exceptions (eg Apple Pay cards, passcode).
  • Unpair your Apple Watch and iPhone - Apple Support
  • Set up your Apple Watch - Apple Support

If the issue persists, I suggest that you contact Apple Support (mail-in service may be available) or make a Genius Bar reservation to arrange for your watch to be inspected and, if recommended by Apple, serviced or replaced under warranty:

• Contact - Official Apple Support
• https://twitter.com/applesupport
• http://www.apple.com/retail/geniusbar/