A "topup" and a "goodybag" are not the same thing.
To verify what you purchased and when, check your order history
In your reply, please copy-paste the last few transactions, with their ‘Date’ and ‘Content’
Each entry should say '1 × topup' or '1 × goodybag'
If it says ‘1 x topup’, then you did not buy a ‘goodybag’
You would then be using your SIM on a “pay as you go” basis, paying for every call, text and each MB of data at these expensive pay as you go rates
Mobile data costs 5p per MB, which will quickly spend your credit.
Don’t worry, this is a fairly common mistake and you’ll get your money back. To resolve your issue:
Ask an agent here, they will refund you the credit as it was purchased in error
When the credit is refunded, exchange the credit for a ‘goodybag’ using this guide
To prevent this happening again, I recommend you simply set your goodybag to 'auto-recur' (like a direct debit), so it renews monthly and you don't have to worry: https://giffgaff.com/profile/payment-details#goodybagsettings
Please also turn "Auto topup" OFF: https://giffgaff.com/profile/payment-details#autosettings