How to tell what app is using data on iPhone 4?

I almost never get near my 1g data plan but have almost doubled it in the last week. I'm not Streaming, or downloading. I have turned off everything I can find. Please help. There must be an Ap I downloaded that is doing it, but I can't figure out which one! I have downloaded a few tracking apps, but they only tell me how much I'm using, not what I'm doing to use it.

Make sure your background apps are closed also.