What does "last month" mean?

Let me explain the situation. Today is Oct 31, 2012. After I got my October pay-slip yesterday I had the following conversation:

Me: Hey guy, I have a question to ask you about the pay-slip of the last month. I think the number is incorrect.
X: It is $10, is it correct?
Me: No, I mean last month (I mean September) not this month.
X: Last month? Okay.

I want to know, did I use "last month" correctly in this case? I used "last month" because this month is not passed yet, so I used it to refer to the month before this month. And if it is incorrect, what word should I use.

Solution 1:

You can reduce the confusion by referring to the payslip instead of mixing in calendar months. Last pay slip or current pay slip and previous pay slip. Pay slip for date ending xxxx. Pay periods do not follow the irregular nature of the months.