I would think it's the arm, too. When you cannot choose your position 100%, when something is itchy or painful, the sleep just isn't as deep. Now, I wouldnt go back to total AP either, that will make it hard for when she's better, but not actually 'train' just now. Go in, resettle, reassure, scratch, give pain meds, whatever, but then go back out and let her sleep.
hugs for you and your LO.