Did you have the mobile data enabled? If so, it sounds like background data. Sometimes when you apps open in the background they can be using the mobile data. The most common are social media apps such as SNAP CHAT, TWITTER & FACEBOOK that deal with push notifications.
It always best to turn off the mobile data connection when not in use. The data connection can be disabled under the settings of your phone or you should have a widget to toggle the data connection off/on again.
Please note that it can take up to 24 hours for a response from an agent, however they are usually a lot quicker (every message you send will reset the timer so please be patient) Agent Replies can be found here: https://giffgaff.com/support/questions I hope you get your issue sorted
@wilma998 Hi welcome to giffgaff and the community
This is when background apps are running and use a small amount of your credit , iam sure if you ask a agent they will as a good will gesture they will refund the 40p ( at there discretion) to enable you to buy your goodybag