What does your data plan allow per month? How much data are you using per day? Which carrier are you with? Anytime you are NOT connected to WiFi and your phone is turned on it will connect to the internet via the cellular network thereby using some of your data. The only way to avoid using some amount of data is to turn the phone off and that would be useless.
It is literally just in the last 2 days tho and nothing has changed as far as I know. I have 750 mb per month and usually only use 300 ish. Since yesterday morning I have used the whole lot! I'm on Vodafone.
I'm in the US on AT&T so I do not know much about Vodafone but can suggest seeing what apps are running in the background and shut them all down. While using the phone during your day, when you stop using an app make sure it is shut down completely and see if it makes a difference in your data usage. It seems like one or more apps are accessing the internet via the cellular network and using up your data. Have you installed any new apps about the time this started happening or have you had less access to WiFi in the last few days? Something has changed in the last few days that is causing your cellular data allotment to be used up more quickly, the trick is to figure out what or why. Let us know if you find a solution.