JESS: IRS.
NICOLE: I’m not IRS.
IRS, the Internal Revenue Service. It is responsible for collecting US federal taxes, and is an agency of the Department of Treasury.
Jess assumes that Nicole is at the diner to investigate Luke’s taxes, or provide an in-house audit. These audits can be selected randomly, so Jess is not implying that Luke necessarily did anything wrong when filing his tax return.
The Annotated Gilmore Girls 