Wrongful treatment to customers

i have been banking with the bank for more than 8 years , and i get my salary from the bank every single month . Today i received an sms indicating that my account is in errors for an amount R100 failure to do that it will be taken to legal action. Yet i get paid the 26th and to why bank charges are deducted late i have no idea. I don't understand why such a threat and this is the second time the bank doing this , first they took me to the lawyers for owing them R2 for cellphone contract that ended years back and when call them to find out what happened they indicated that they were not able to get hold of me yet i been getting paid from their account for years . I feel FNB doesn't care about their customers, i am even looking at the option of changing the bank.