Demanding money 7 months later or threatening to hand me over

I left the Fedhealth medical aid scheme 7 months ago, I get a letter from them 7 MONTHS LATER to say that I owe them R4 500, 7 months later I get this letter, I then tell them that I need to make a payment arrangement as I am a single mother and I cant afford to pay them this money, they sent me a letter demanding that I must pay R600 per month, I then wrote back to say that I absolutely cannot afford an installment of R600 a month, and they are refusing to make an arrangement with me that suits me, 7 months later they want to demand money from me, and threaten to hand me over, if they had contacted me in the month that I had left Fedhealth to settle outstanding monies, but not 7 months later and to demand that I pay this money when I am saying to them that I cannot afford to pay back so much..