My iPhone shuts down when the battery is still 18%, what's wrong?

Sooo my phone started to shut down sooner and sooner (5% - 9% and now 18%).

I usually charge it every night and in the morning it says 100%.

Also, the battery percentage drops a lot in a short amount of time.

I don't use my phone excessively or anything so I don't understand why it would just shut down like this.

- iPhone 4s 16GB with iOS 6.1.3

Please help.