She danc'd along with vague, regardless eyes, Anxious her lips, her breathing quick and short: The hallow'd hour was near at hand: she sighs Amid the timbrels, and the throng'd resort Of whisperers in anger, or in sport; 'Mid looks of love, defiance, hate, and scorn, Hoodwink'd with faery fancy; all amort, Save to St. Agnes and her lambs unshorn, And all the bliss to be before to-morrow morn.
Madeline is definitely a little out of it: she's physically present, but her mind's elsewhere.
Also, when we say physically present, we don't just means she's there in the hall dancing, we mean that Keats is making a point of her physicality—he talks about her lips, her panting breath, her sighs—but then makes it sound like she's not actually sensing the stuff that's actually around her (the whispering guests, the drama-rama with the loving and scornful looks). Instead, she's focused on the stuff that she's anticipating at the "hallow'd hour."
In fact, despite all of the breathing and dancing she's doing, she's described as "amort"—which means dead—and, perhaps even weirder, as "hoodwink'd with faery fancy."
So this is the second time we've seen the word faery in this poem (remember "fairily" in line 39), and we're only eight stanzas in. Hmm… it feels like "faeries" are becoming a theme here. In any case, Madeline is metaphorically tricked ("hoodwink'd") out of her right-thinking mind by this fairy influence.
The "lambs unshorn," by the way, tie back into those rituals that Madeline thinks she has to complete if she wants to make the presto-magic-vision thing work. It was tradition to bless two white, unshaved lambs on St. Agnes' Day, and then to spin and weave their wool.