If you got your phone pretty early on, like me, then it's 2 years old or a fair bit older! I've noticed that my battery has been lasting less and less time since a couple months ago. Now I have to charge it two or sometimes three times a day, if I actually want to do anything at all with it.
I'm sure the battery needs changed, but since my apple care expired I've been trying to wait until a new iPhone is released. If there's ever going to be a new iPhone, that is ...
If I didn't have so much money invested in ******* apps I'd switch to Android.