My iPhone 4's battery is starting to drain quicker?

I have an iPhone 4, iOS 6. My brightness is always on the lowest possible setting, I regularly shut down running apps, I don't use any push notifications.
I had a full charge this morning and within two hours it was down to 65%! And I had only even touched it a couple times to see what time it was! So I charged it in math class. Within four hours of again hardly using it, it died. My battery has never done this before! It's only ever died on me once before!
I've deleted some songs, a couple apps that I don't use, and a ton of pictures but my battery is still draining so quickly.
Arrrg! What's wrong with it? Will I have to get a new battery? Or is it something I can fix?

I've noticed that iOS6 does drain the iPhone 4 battery more than the old operating system, but if you've had the iPhone for a while, and haven't always charged it responsibly, that may be the reason. It may be getting old and you may finally be seeing the effects of a lot of use. If it's new, you should be concerned. However, it sounds like the battery has just gotten old, and when that happens there's nothing you can do to reverse it besides getting a new battery or phone.

New battery!

You might need a new battery. Did you want to charge it until it was (nearly) fully drained? That's not good for batteries.

I recently read about the advice to always have your phone on the adaptor when you're at home so that it's always charged.

Apparently it's not good for batteries to be completely drained before charging them.

As for your current one, I think you need a new one. Or the battery indicator is broken.