If you are using Windows Phone 8.1 Developer Preview check your phone for an update. Microsoft have rolled out the first set of updates for the phone OS. The updated build is 8.10.12382.878 and Microsoft say there are fixes in response to user feedback and improved battery life which is probably the biggest criticism of the OS so far.
To get the update go to settings, phone update and check for settings. The update doesn’t seem that big, my HTC 8X updated in about 20 minutes.