I am having same problem.  In November, i got an alert that i was going over my data plan.  they told me time and date of large chunks of data being used.  

Same thing happening again this cycle.  

The common things I've noticed is that Ive physically been driving in a certain area when it happens.  

My usage prior to november 2012 was consistent at below 1G.  I dont stream music, do video or anything.  now all of the sudden, I'm up to 3 plus gig used.  

I have no idea what to do and ATT doesn't seem to have any answers.