Why is my iPhone 4s running out of battery so fast?

Obviously iPhones are renowned for low battery lives. But mine is normally fine, i turn my apps off, brightness is never usually high. Though starting yesterday it has been dropping ridiculously quickly, going from 100% at 9am and then within the hour it's gone to less then 30%, my battery isn't lasting longer then two hours before dying, previously it had lasted as long as 8 hours and is heating up alot, can anyone help me out?

Cell standby
brightness

Get a new Phone

It could be one of these: Running too many apps in the background
Using Bluetooth when you don't need to use it.
Keeping your 3G on when you don't need it.
Vibrations

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