If she actually DID pay at the end of the month, that would be another thing entirely, wouldn't it? She has just paid September's and we're halfway through October. So she's been six weeks in arrears.
How is the 14th the end of any month on the calendar?
What she means is "I'll undertake, as usual, to pay you at the end of the month but the string it out, as usual, and not pay until I've made as much drama as I can handle."