Raised installment

Good day. I am dissatisfied with the customer services and explanation by foschini.i am a pensioner and has had this account for years and then last year for a couple of months I could afford to pay more on my account all of a sudden my installment is raised to r1100 and when when I went to pay in Jan r500 I was told my account is in arrears.when I called the call centre to enquire I was first told that it is because I'm on a six months payment plan and interest accumulated .so was advised to change to a 12 months .and I needed to call collection. When I phones collections I was told that I'm in arrears because I paid so much the previous months and that my installment has always been so high which to me was a surprise and trying to explain that this is not the case and the customer care lady never said anything I was told that they should have explained and that being on 6 or 12onths won't make any difference. So how can an account that has never skipped a payment and been paid even more than the installment be in arrears because you paid to much.so as a pensioner I must give my whole r1100 grant money when I've been a loyal and constant payer this is the treatment you get.