The time/date stamp on usage provided by the carrier is not accurate. No carrier time/date stamps data usage realtime, it is batched.
Are you stating that the data usage on the device has gone up since turning off Cellular data on the device?
Check the outboxes for each of your email accounts, it is likely that an email is stuck there trying to send and running up data.
You were right, there is an email with several photos stuck on send.
I can't find a button to cancel the email though.
I reckon it's been there for a few days indeed!
Edit: I was able to cancel that email. It's been in there for 5 days, trying to send those pictures. I'll leave my 3G on now for a few hours & see if my data usage skyrockets more.
Thanks for your suggestion