Can I legally be held to this?

[EKnight74]

I live in South Carolina-- Horry County-- and borrowed around $600 from my mother six months ago, with an understanding that the debt was to be paid off in increments whenever I could manage. I have paid back $200 of it, but she and I recently had a huge disagreement, and she has decided that she no longer has any interest in even acknowledging my existance, except to inform me that she is demanding the amount to be paid in full by April 1 or she will sue. I find this to be nothing more than petty spitefulness on her part, as she knows that I was just laid off from my job, but in the unlikely event that I would be unable to repay the loan by then, can she legally demand the amount in full even if I have in writing from her a repayment agreement that states, "will be paid whenever"?