Scene 6: What reasons might explain Blanche's apparent depression at the beginning of this scene?

We learn in the stage direction that Blanche is exhausted.... likely because of a medical condition called neurasthenic personality, which is "an ill-defined medical condition characterized by lassitude, fatigue, headache, and irritability, associated chiefly with emotional disturbance."

It is about two A.M. on the same evening. The outer wall of the building is visible. Blanche and Mitch come in. The utter exhaustion which only a neurasthenic personality can know is evident in Blanche's voice and manner. Mitch is stolid but depressed. They have probably been out to the amusement park on Lake Pontchartrain, for Mitch is bearing, upside down, a plaster statuette of Mae West, the sort of prize won at shooting-galleries and carnival games of chance.