Serious problem: battery discharging itself.

Hello, I have iPhone 3GS for 2 days now and I've noticed that the battery is discharging way too fast. After first night I noticed that the battery is on 0% and requires recharging. Then I realised that I had more than 10 applications running at the same time (I didn't know how to turn them off at that time). Next night I was sure all the applications were off. In the morning the battery was on 0% again. And now 1-2 hours ago the battery was on 65%, after 1-2 hours it was 50% despite of the fact that nobody used the phone in ANY way.

I have double tapped the main button to see if there are any applications running, but there were no applications running. Any ideas why is the battery running low so fast without even using the phone?

Thanks!    

Hey i would like to advice u..
Recalibrate your battery meter: Fully discharge your phone until it turns off on its own, then plug it into a wall outlet and charge to full.
thanks for sharing a great information on the topic.
thanks.