Unfair treatment from Lancet

I went to a Dr and did bloods which were not covered by the medical aid. The dr then forwarded me the bill incl the bank details. I have been paying this account monthly, without any communication from Lancet. I finally got a statement last week notifying me that my balance due is R150. Within getting this SMS, I then got harassing sms, calls etc threatening to hand me over etc. I have been religiously paying this account for months and no one ever made contact, even last month I paid it, now that I am left with 1 installment I get such communication from Lancet