Why does my iphone keep going over my data?

I check my data at least once every day or every other day when i go on it when i'm not home.today i checked it and i wasn't on anything and has everything closed and it told me i used 188.02 of 200. I got home about an hour later and i got a message from att saying i went off my data by 7. I don't know how it keeps doing this. Or why? When i'm home i'm connected to Wi-Fi. Why is it doing this cuz i don't want to bay a $300 phone bill again

Added (1). Iphone 4

Do you have the 4, 4s or 5? Turn your cellular data off. The 5 uses way more. You need atleast 2gb not 200mb

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