RORY: Maybe I didn’t study hard enough. Maybe I got cocky.
LORELAI: Maybe you need a major mud bath/salt glow/chill pill combo.
Take a chill pill is American slang meaning that the person addressed needs to relax. To chill means to act cool, and the concept of chill pills probably comes from drugs like Xanax, Valium, or even Ritalin, as if the person needs to start acting as relaxed as someone under medication. It dates to the late 1980s, with use peaking in the 1990s.
Lorelai humorously suggests that Rory should combine her “chill pill” with relaxing spa treatments such as a mud bath or a salt glow because she’s acting so stressed that a simple pill won’t be enough.