As an agent (called an attorney-in-fact in some states), you are duty-bound to avoid any sort of personal benefit from financial transactions you conduct on behalf of your mother-in-law -- and to keep your own finances separate from hers. Your best intention might not have been to stick the woman with the debt, but I am afraid that what you have done is illegal as can be.

Some power of attorney documents explicitly waive these requirements -- most often when the agent is a spouse or life partner -- but most do not.

Do pay your mother-in-law what you owe her as soon as possible. Do not use her credit card to do so.