Data usage on a Wi-Fi connection

I got my iPhone two days ago and I've been careful about using data. So far I've been using a wireless connection to use apps that need internet, and I've only used my 3G data once to check when a store opens/closes. I always exit out of background apps right after I use them. I got a text this morning saying I've used 75% of my data (I'm on a 100MB plan), but I've only used 3G data for a good 1 minute. What does this mean? Help! Thanks in advance.

I'm with Telus (Canada) if that makes any difference!

I'm sorry but if I were you I would get at least a 1gb plan… Smartphones use data all the time for no reason

You may not realize that quitting an app will not completely stop an app from using data if the app is set to constantly scan your location or if there are any replies, or messages, etc. Is Setting (app)/Location Services turned ON? How many are turned on, how are they used? Does an app constantly check the internet for anything?

All of this is totally separate from turning off an app.

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