It appears that you may be the victim of number hijacking. This can only happen if the other person gained access to your giffgaff account, so you should immediately change your giffgaff password. You should also change your password on any other site where you use the same email and password combination. NEVER use the same password on more than one site.
As a precaution, you should also alert your bank to the possibility of fraudulent activity - especially if your phone number is used to receive One Time Passcodes.
Did you receive any unexpected emails or texts from giffgaff saying your number transfer or SIM swap was under way?
Is your number still shown on your giffgaff dashboard? https://giffgaff.com/dashboard
Are the correct name, address and email details shown on your account?