Instalment

I have credit card with woolworths that I have been paying on the 20th every month all of a sudden I received a call they said I'm on arrears. I must make payment I asked them how because I have been making payments on the 20th how did my account fall on arrears? Their answer was my credit they balance it on the 13th so I must make payment before 16. I asked them but when I app**** I told them I'm getting paid on the 20th why all of a sudde? They couldn't answer as a result exeprian is showing I am on arrears now because of that and that's affects me big time. I am really not happy