Is your mobile data enabled? As this can be associated with background data. Apps running in the background can occasionally make use of the mobile data or it could be apps being updated in the background.
It is recommended to switch off the data option under the device settings when the mobile data is not in use or update apps over a WiFi connection only.
It could be a case that your phone has background apps/app updates running that could be using your goodybags data very fast. It is worth checking and turning these updates off and seeing if that helps reduce your data usage.
See this link for a guide and tips for data usage.
Sorry for that. Smart phones update there apps in the back ground and uses data unknowingly. However, you can check the data usage on your phone by going to Settings -> Mobile -> Mobile Data -> Mobile Data Usage to see how much mobile data used and what apps have used that data. You can also check data usage from within the giffgaff app if have installed on the phone Or can check the usage details by sending text PLAN to 43430 you will receive a text with usage of past 30 days.