Whilst I'm afraid that I can't say why that happened, the following steps may help to prevent it from happening again:
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
If you are sharing an Apple ID with another user, then stop doing so.
- For Activity tracking and reporting and various other features and services to work as intended, each person should each have a separate Apple ID and be signed in accordingly on their own devices.
If you are using any third-party fitness tracking, sleep tracking and/or heart rate tracking apps, then:
- Check within those apps on your Apple Watch for any active or paused sessions and, if you find any, end them from within the app on your watch.
- Check the developers' support resources and/or contact the developers to check for any known issues affecting how data is shared to the Health app on your iPhone.
- If in doubt, consider removing any such third-party apps from your Apple Watch and/or iPhone.
Close certain apps and then restart (normally) both your iPhone and your Apple Watch:
- Close the Watch app, Activity app and Health app (if open) on your iPhone.
- Turn both devices off together, then restart your iPhone first:
If the issue persists, next 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
If the issue then persists, I suggest that you contact Apple Support (remote diagnostics and/or mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider for assistance: