Phone Downloading GBs Data over 3G in background even when on WIFI iPhone

Yesterday I started getting texts from Vodafone to say my data usage is up and I will be charged £5 per additional 500Mb. I thought nothing of it as I was not using the phone, and it was connected to WIFI. Then I got a message to say my additional 500Mb is up. Then another later on. I checked my Usage and it stated 2.1Gb. I reset this to 0. Then in the middle of the night I got another saying my 500Mb is up. I checked usage and it states 500Mb used since last rest. I have not been using it, it is on WIFI!
I have not added any new apps. The only thing I can think of is Evernote but then I only have 55Mb of notes to sync. so that cant be it.
Does anyone knoe what this could be?? I am being charged £5 per 500Mb, thats £15 so far since last night and I have not done anything!
I restarted the phone and its not doing it now. But I want to identify it because I dont want it to happen again. I have no idea what 2.5Gb of data have been downloaded to the phone.
Thanks
iMac Mac OS X (10.6.3)