It’s likely you have been accidentally purchasing ‘credit’ / ‘topup’ instead of a ‘goodybag’. To verify this, check your order history
In your reply, please copy-paste the last few transactions, with their Date and Content
Each entry will say 1 × topup or 1 × goodybag
If they all say ‘1 × topup’, then you did not buy a ‘goodybag’ as suspected.
You would then be using your SIM on a “pay as you go” basis, spending your credit on every call, text and MB of data at these expensive pay as you go rates
Mobile data costs 5p per MB, which will quickly spend your credit. For reference, £10 ‘credit’ is equivalent to only 200MB data, compared to a £10 ‘goodybag’ with 6GB data (6,000MB).
Try to resolve your issue by asking an agent again, explaining that you did not understand the difference. They normally refund you at their discretion:
Request for a refund by asking a giffgaff agent
When the credit is refunded, exchange the credit for a ‘goodybag’ using this guide
To make your life easier and prevent this happening again in future, I highly recommend you simply setup your goodybag to automatically recur each month.