Check your current goodybag allowances here: https://giffgaff.com/dashboard
Please note the amounts shown are what you have REMAINING, not what you have used.
Note that giffgaff merely provides the mobile data. How much data your phone uses is down to the phone's settings, NOT the network. If you used a different network SIM with roughly the same network speed, then your phone would have used basically the same amount of mobile data.
As long as your phone has an active mobile internet connection, it will continue to use data in the background, even without you actively using the device.
You should work out where your data has been used, narrow down which apps are actually using your data, for example:
On Android: Settings -> Data usage
Or install a separate data monitoring app
Once you work out the offending apps, try to configure them so they update less, or over WiFi only. Or even uninstall them if you don't actually need them