In daily training, Mulan isn’t just hampered by her inexperience and her (relative) lack of muscle.  

Though she’s capable of calm thinking in chaotic circumstances, she’s never been in this extreme of a crisis before and so panic over exactly how ill-equipped she was for the life of a soldier sometimes overwhelms her.  (As she grows accustomed to training - both psychologically and physically - this lingering fear will become less rational and eventually pass.)

She is also concerned that the thick binding she wears under her jacket hampers her movements; at the very least it soon becomes damp with sweat and chafes her body.  Mulan knows that it’s either this or leave her breasts loose under her man’s jacket - that is, open herself to imminent discovery - but that doesn’t mean she enjoys it.