How to stop my iPhone 4 from using 3G?

I have an iPhone with an AT&T plan. I don't know how but during school I wasn't using my phone at all and when I checked my phone after school there was a test message saying I have used up all my 3G and will be charged if I overspent (I have limited 3G Wi-Fi). How can I disable the 3G until I get more data for it. And I overspent like 4 times already without using my phone at all! Do you know why this happens? How can I make it stop overspending my 3G if I haven't been using it?

You go to settings- general-network and there you should have the Internet controls.
You can just disable the 3G or the Internet connection completely from there, which is called cellular data.
Hope it helps
Also check if you have auto updating for e-mails as that can use Internet on it's own and will do so every so often.

You could turn it off by going to Settings > Cellular & switching it off (assuming you are running on iOS 7).

As to why, assuming you are not using a Wi-Fi connection at the time, the phone does background things as well like email, checking other internet based services you have installed, etc. All smartphones operate like this. Again if you are on iOS 7 you could disable what apps use the 3G connection.

Oh & there's no "3G Wi-Fi", they are 2 separate but related things.

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