I had a very similar experience a few days back i noticed my Pebble (TR) stopped vibrating.... COMPLETELY.
There is no pulse at switching motion backlight on or at switching it back off. No vibration at switching the volume up or down too. Not to say the 10,000 other minute places, including calls or even scrolling through in the vibrations menu.
Dont know what to do at all.
Bought it a year ago. Just lived well enough till the end of its warranty life.