Debit order not taken now arrears are what I pay

I have had my gym membership for some time and although I have only used the gym like for 6 month I still get deducted at the end of the month which is fine,earlier this year I called to find out how much it would be to cancel my membership and I was told R1100(that included arrears) which was fine and mind you that was in February. A couple of days ago I was contacted about my arrears which was now R791.00 at the time I brushed it off but then I went through my bank statement and saw that the last time a payment was debited was in June. I then spoke to the lady from Gym company via Whatsapp who said that there was no money in my account but what I do know for a fact is that on the last day of the month when the debit order does go off it goes off in the morning before I can even withdraw money so how was there no money and even if there was no money,was there no money in my account for 3 months(July, August, September)? I have now been told that I would need to pay this amount of R791.00 or I will be blacklisted I am aware that in February that my account was in arrears of R300-/+ and I will not deny that but the amount of R791.00 of arrears cause the debt order didn't go through doesn't sound right.