Why are the nuns worried about Cyrano?

They worry that he is so poor and won't let anyone help him out financially.

They worry because he is poor and is probably not eating. To hide it he teases the novice by claiming to have eaten meat on Friday; a day good catholics should fast and eat fish inseatd of warm blooded flesh meat. When the novice complains about his lack of piety to another sister, the sister explains that it is bravado and he probably hasn't eaten at all.