Medical Aid

I was member of a medscheme, before my membership was terminated I had funds in my medical aid which I used them. Now I received threatening letter saying that I must pay them or I will be blacklistef

My question is one to, when my account was closed they could see pending payments but bear in mind that that I had funds. No provision for the payment to service provide and now I must pay them. Why because I had funds in my medical aid when the transaction take place.