Horrible service

I want to express the horrible service from FNB. I have a loan account and I am currently on maternity leave. I've made an arrangement with them to pay the installment by the latest the 10th of each month! I have been paying that installment on time every month. They have decided to increase my instalment by duoble the amount without letting me know and they have deducted more than 3 times the instalment this month!! That is unacceptable!! What is the arrangement for then! I am seriously considering moving my account to another bank that has far better customer service. This is really unacceptable!