Nedbank just takes my money as they want

I have made an arrangement to pay off my small personal loan. I have kept my promise and have not missed a payment. Nedbank, legal, decide to take any amount ranging from R200 to R1500 whenever they feel like it. I am a disabled pensioner with Multiple Sclerosis and I am sick and tired of Kelly Britou and her emails. She also told me on the phone in front of a Nedbank employee that \as long as you owe nedbank