My mom is obsessed that she does not have enough money. Why?

Nothing could be farther from the truth. Was hospitalized years ago for this. Over 3 years ago my mom started to exhibit very odd signs. She was paranoid and thought police would come for her because she was convinced she owed money to people. She COULD NOT BE convinced by the rest of our family that this was not the case. She was so CONVINCED that she had no money, even though we showed her the bank account statements. She was hospitalized for a few weeks at that point. She spent time in a regular hospital and rehab. after that and miraculously showed lots of improvement. However, my brother said these signs are starting again for no obvious reason we can understand. Is there something you can explain as to what kind of sickness this is due to an obsession with a "lack of money"?