Taking more money than is due to them.

I have paid my loan in full according to their app and yet they continue to debit my bank account every month. Then when I call or email to find out what is happening, they tell me that they sold my loan to another company but fail to give me any appropriate details as to who this company is. Also saying that I defaulted on payments when all my payments were made on time and reflected on their app. I swear I will never use them again.