Poor record- keeping

Good Day my name is Rain-Linda Makhanya I received an sms that I need to make a payment for this month being May, whereby I had made a payment for end of April being R505 for the month of April, on the 10th of May I made R1000 in-store payment for the month of May, when I called the call centre as to why I need to pay R505 on the 30th of May as payment for May has been received the R1000, he told me the payment cycle starts end of the month till the 10th of each month. Told him that's bull**** I don't remember receiving any memorandum that entails such. There's is no way I'm going to pay that R505 for the month of May whereby I had already paid a R1000 for May. You guy's need to update your records cause it's just bull****. Meaning in total need to pay R1500?? I hope it gets sorted out and it better not affect my credit score. Failure in rectifying I will take it with the Ombudsman