Distasteful customer treatment

I have been a Woolies customer for a while (years) now and I pay my account monthly. I have recently been unemployed and therefore not able to pay most things right now. I had to pay about R 250 or less in Nov. I received a call from their call center reminding me to pay, I told the lady on the line that I am financially strapped and that I'd pay once I get the cash. She said she's make a note on my account. Suddenly I get an sms that I have to pay R 426 the next month, and endless calls from their call center that disconnect every time I answer. Now I have to pay R 638.71 or else I'll get negative credit. How do I get negative credit when I've been a good customer for so long and had explained my situation but the store card amount seems to spiral weekly. I pay my account monthly and hardly exceed my limit or take things on credit.I am really at my wits end and disappointed at the handling of this situation. It's unfair.