Unknown data usage
I have the 2gb plan. This has worked fine for a year until yesterday when our wifi went down and I turned off wifi so I could get my email. I forgot to turn wifi back on...big mistake.
Understand that I am only 15 days into my 2 gb. While i am asleep at 2:41am a message came saying I and 20% data remaining. At 3:05am, 9% remaining and at 3:37am 1%. In other words, while I and my iPad were asleep, it used 400 mb of data in one hour.
After turning wifi back on and renewing my data plan, I looked at what was running and closed everything out. The problem is that I really don't know what app was draining my data. The moral to the story is close your apps and turn it off (not just sleep) especially when not in wifi.
This is certainly a puzzle because the 'usual suspects' such as downloading movies, music or big apps simply can be to blame because Apple limits all such downloads to 20MB. Do you have streaming video or music players?
I had the same problem with a Telstra iPad 3G data plan here in Australia. They tried to tell me I had downloaded 2GB of data in just over 4 hours!
Needless to say I argued my point and as they weren't able to explain the data that was used or tell me which web sites it had been coming from they credited the data back onto my account.
I'd query your provider but I'm also now very careful t turn cellular data off when I don't need it.