IPhone battery is going flat too quick?

I've had my iPhone 4 for about 5-6 months now, and I've only noticed today that my battery has gone flat very quick? I charged it overnight, then I used it at lunch at it only had 40% battery, then when I went to get it out after work and it was flat. Any suggestions as to why it may be doing this?

It happen to me right when I started using photo stream so I stopped using it and my battery is working great now. I'm not sure if anything changed with you or not. If that's not the case make appt with Genius Bar coz you have one year warranty they can fix it for free

Looks like you need a new battery

If the phone is already several years old, it may just be an aging battery. The battery gradually loses capacity over time, whether it is used or not.

If the phone is new, chances are that you are running things on the phone that use up a lot of power. Anything that plays music or video, or uses geolocation, or communicates over the Internet (a cell data link uses more power than Wi-Fi, but they both use a lot of power), or does anything in the background will consume power. Games use a lot of power, too.

So either the battery is old, or something on the phone is using power. In the former case, the only solution is a new battery (they are easy to replace at an Apple Store).In the latter case, check to see what you are running and turn off an apps you don't need. Also, make sure you set your phone to go to sleep after a few minutes. If you leave it on all the time, it will chew through batteries very quickly. And consider turning down the screen brightness, if you have it turned way up, or set it to automatic with a low preference setting.

Depends on brightness and other things.

My battery has been fine for years now, may be something you are running in the background?