Accused of being in arrears

I took out a loan with capitec in December 2015 and I was out of the country for the past two weeks. As I was away i kept on receiving sms that my account is in arrears, how is that possible, when I communicated to them that my bank account can be debit on the 18 December 2015 which was the pay day in December as the 25th is a public holiday. Today I was forced to agree that my account is in arrears in order for them to transact the debit order from my account with immediate effect.