The TSA has a help line for passengers with disabilities or medical conditions (including lactating) who want help with the screening process. If you call them 72 hours before your flight, you can request assistance from a "passenger support specialist" or just talk through the process of transporting your milk through security.
TSA Passenger Support
You can also print and complete a TSA Disability Notification Card and present it to the officer at the screening point. While you still need to go through the screening process, it is a discreet way to let them know you are carrying breastmilk.