As you only purchased 'credit', you are then 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).
Don’t worry, it’s a fairly common mistake and you’ll get your money back. To resolve your issue:
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 also recommend you simply setup your goodybag to automatically recur each month.