What do you mean by credit?
Cash credit or membership/add-on allowances?
If you mean cash credit then you control that and can see how it was spent on your My History page:
The only other way for cash credit on your account to get used up is by using services that are not covered by membership/add-on allowances and are charged to your cash credit balance - e.g. international calls/texts, MMS messages, calls to "special" numbers such as 1850/1890 numbers, non EU roaming charges etc.
If you mean how did your membership/add-on allowances get used up then your phone's call/text logs should indicate how call and text allowances were used up and your phone's data log should indicate what apps are using how much data.
If you need more detail than that then you'll have to contact 48 support and ask them if they can help.
Hope this helps.