IPhone 5 shutting down way too soon?

Ok so for the past few days my iPhone 5 has been shutting down at anywhere from 16-20% battery level. This is annoying and very bad for me because I rely on my phone a lot.

Why is it doing this? It wasn't like this before.

It'll get to about 16% usually and turn off as if the battery level hit 0% and when I try to turn it on shows me the red battery screen and prompts me to connect it.

Restart it or restore it.
Normally helps with alot of problems.
Otherwise call apple.