Hi
Whilst I can't say why that is happening, your Apple Watch may require battery service.
Before contacting Apple, the following steps may help:
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
Install any available updates for apps on your iPhone.
Check that you are following Apple's charging instructions, including:
- Checking that your Apple Watch is connected to the charger, correctly aligned and touching it fully, and connected to power.
- Using original Apple or MFi-certified charging accessories.
- If you are already doing so, try using another such cable and/or power adapter.
- Checking that all wrapping has been removed from all items, including checking carefully on the head of the charging cable.
- Checking that the charging cable and power adapter are fully inserted into the port and power socket.
- Check your battery and charge your Apple Watch - Apple Support
If you are using Nightstand Mode / Bedside Mode, then, when placing your Apple Watch into that mode:
- Check that your Apple Watch is standing on its side in a fully upright position (at 90 degrees to the surface), with the buttons on top.
- If you are using a third-party accessory such as a charging stand, then it may help to remove your Apple Watch from it and to instead place your Apple Watch directly on the table.
Restart both your iPhone and your Apple Watch, turning both off together and then restarting your iPhone first:
- Restart your iPhone, iPad, or iPod touch - Apple Support
- Restart your Apple Watch - Apple Support
- If the issue persists, next close open apps on your iPhone and then restart both devices again.
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.
- When setting it up again, initially choose to Restore from Backup. Most data and settings will be restored, with some exceptions (eg 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
Having tried the previous steps, if the issue persists then I suggest contacting Apple Support (remote diagnostics and/or mail-in service may be available, if required), making a Genius Bar reservation or visiting an Apple Authorised Service Provider for assistance: