I have an iPhone 5 for AT&T, why did i use all my data (300mb) in about a week?

Okay so i bought my iphone 5 a week ago and 90% of the time i connected it to the Wi-Fi at my house and i just got a notification saying i used my 300mb and i know thats inpossible, what should i do? Doesent the iphone automatically just use the Wi-Fi network and only the Wi-Fi when its connected to it? Please help, i don't wanna give At&t another 20 bucks

Turn off "push technology" and bluetooth

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