iPhone does not show battery level correctly

I have an iPhone 4, iOS 7.1.2, and the battery level isn't showing up correctly. When I'm in an app that doesn't show the top bar (time, cellular signal/wifi, and battery level), when i go into another app or the home screen the battery level goes down dramatically. But when I plug it in, the battery level increases by around 20% as soon as I plug it in. Is there a way to fix this? Thanks.