I think you should inform the shelter so they can start measures themselves.

After getting back from the vet my kitten seemed unusually sleepy.
She is usually bouncing around, taking light catnaps occasionally.
Well, she slept long and sound, not even awaking when I waved my
dinner in her face.  She eventually came back to life later in
the evening.

She had gone to the vet and was given grisofulvcin for ringworm
and Capstar for fleas (Capstar was mandatory for all patients).
The data on grisofulvcin made no mention of drowsiness, the info
for Capstar mostly said limited side-effects but I did run
across a mention of lethargy.  She also had a bath/shampoo and
I am wondering if the staff possibly slipped her a sedative
(no mention of it nor charge on the bill).