Do you have auto top-up enabled? This is where your credit gets topped up when you fall below £3. You can check and change it here https://giffgaff.com/profile/payment-details
Please also check your order history to see what happened. https://giffgaff.com/orders/history
Dial *100*7# to check your goodybag status.
Dial *100# to check your credit balance.