harrassment

i have been harassed by truworths over a payment. In November I took clothes to the value of 240. and my monthly repayment is suppose to be R40. and end of November I paid 200 which is five months in advance. January I received a call from truworths saying im behind with payment. I've been receiving their calls and emails 3 day intervals . and they have added interest on my account as well. why truworths cant distribute this amount over five months. a