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.
If you feel this incorrect then all i can suggest is you ask a agent to look into this. To contact a giffgaff agent please follow this https://giffgaff.com/support/questions