“One grub too many”

SOOKIE: You just had one grub too many. Just drink lots of water to rehydrate.
JACKSON: I will.

I think Sookie is referring to mezcal, a Mexican spirit distilled from the agave plant, similar to tequila, and extremely intoxicating. Some brands include an edible mezcal “worm”, an insect larva which is the caterpillar of a moth, or occasionally the larva of a weevil, both of which naturally infest agave plants. The manufacturers added it in the 1950s as a marketing ploy. I think this is the “grub” Sookie means.