Apple Watch series 5, iOS 9.3.1

If all settings normal but behaviour is irrational there is a progressive hierarchy of remedies. Hopefully you will not need to go to the end of the list.

First force restart the Watch

i.e. press and hold both the side button and Digital Crown for at least 10 seconds, then release both buttons when you see the Apple logo.

 Then, although it may seem irrelevant, while it is restarting Force Restart the paired iphone   Force restart iPhone – Apple Support (UK)

You may also want to make sure the software on the watch and iphone are both up to date to the same level. Updates are in pairs.

Update your iPhone or iPad - Apple Support

Update your Apple Watch - Apple Support

Test it for a while. If it is still misbehaving... Unpair and set up again

 Unpair and erase your Apple Watch – Apple Support (UK)

and then set up if necessary without restoring the backup - takes about 20 minutes

Set up your Apple Watch – Apple Support (UK)

If you've completed that troubleshooting and the issue is still not resolved, Contact Support.

 Click here > Contact - Official Apple Support

or phone numbers 

Click here > Contact Apple for support and service - Apple Support

and or Genius Bar Reservation and Apple Support Options - Apple