Credit facility

i received a call telling me that i have not made an payment in May and some other month. now what i do not understand is how i can continue using the credit facility if i am behind with payments and how is it that once i make payment it does not backdate the payment to the months i have skipped. the fact that i can still use my credit facility as a customer i am under the impression that my account is up to date. i need clarity on that and if it is stated on the terms and conditions i want to see it.