Cellular usage being used while I'm sleeping and on wifi?

I have a iphone 4s on Vodafone's pay monthly.
While I am using wifi, my Cellular is being used and it's going up every 5 minutes by KB not MB.
But as you know KB will eventually turn into MB and one day this will have me going over my Data limit (250mb).
I've only tried Double clicking my home button to see if there are any background apps.
I can't think of anything else to try to find out what is using my Cellular data!

Ps: I have push notifications enabled on Facebook/Instagram/Twitter etc but even when i'm not recieving them, something is taking my mb.

you may have a little problem. Trying to shut everything down and restart everything over. Or call customer services

Certainly your notifications are taking up the data… Try switching push notifications off for one night… And then check the data usage next day…
if this doesnt help, you can switch off your data n wifi connections manually while you sleep

If your Wi-Fi signal drops out occasionally, then it will default to your network data while it tries to reconnect. Ensure it is turned off and just the Wi-Fi is on. If it still happens you may have inadvertently downloaded some malicious software.