How did I go over my cellular data?

I have an iPhone 4. This morning I got a text message telling me I used up 90% of my data so I went over to settings and turned off cellular data as you can see in the picture. Yet, I still got a text message 4 hours later telling me I just went over the data by 12 mb. HOW IS THAT POSSIBLE? I had already made sure I turned off cellular data hours before that. I don't get it. I have AT&T btwHow did I go over my cellular data

You could check to see what apps used data under Usage. Also in future could look into getting data monitoring apps.

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