IF that's the case, it could be that your app is running in the background.
1. do a hard reset by holding the home button and the power button simultaneously for 10-15 seconds until the apple logo appears (ignore the red slider), and see if this fixes the problem. (* the usage time and standby time will appear -- on them if you do this and you will have to fully charge it to get the readings again), i suggest doing a hard reset when your phone is fully charged, also after you do a reset, let the phone sit still for about 5-10 minutes of doing nothing (after you unplug it). Observe the usage and see.
If the problem still persists then go to Settings > General > Reset > Reset all Settings (This will NOT delete any data, all this does is resets all your settings). Some people who had the issue like yours went this route and actually fixed the problem.