There might be the problem of certain apps. Some apps will run in background even you manually close it at multitasking bar. A hard reset may solve the problem.

Hard reset: Press-hold the Home button (big circle below the screen) and the Sleep/Wake button (on top of the iPhone) simultaneously; Continue holding both buttons (Ignore the "Slide to power off") until the iPhone shuts off and begins to restart; You may let go when you see the silver Apple logo.