Retha explains Cassia isn't here. Since the area is locked down, getting to her is not possible.

She appears to have suffered from a common stomach bug, but this is virtually unheard of for the Draconae. Retha will show them bottles of wine and other items she interacted with.

Curiously, while she was found ill in her dressing room a few nights ago, Castiel won't smell or find any traces that point towards a typical illness, suggesting that what affected her must've been magically induced.