Sometimes, you can experience unusual battery drain on your watch. It can be for a variety of reasons. HERE are steps that you can work through to check to see if one of the settings is causing the problem.
This all said, it is sometimes quicker and easier to perform a Master Reset.
To perform a Master Reset:
Press and hold Up/Menu button (middle left)>Up/Menu button to 'System'>Select button (top right)>Up/Menu button to 'Reset'>Select button>Up/Menu button to DELETE DATA AND RESET SETTINGS>Select button>Down button (bottom left)>Up/Menu button to show 'Yes' in the middle line>Select button
Once reset, you will need to set the watch up again and treat it as a new watch in order to connect to your smartphone and Garmin Connect. To acquire the GPS time, you will most likely need to take it outside to give it a clear view of the sky.