... Harry Knew why she wanted to spin out their time on the riverbank; several times he saw her look up eagerly and he was sure she had deluded herself into thinking that she heard footsteps through the heavy rain, but no red-haired figure appeared between the trees. Every time Harry imitated her, looked round (for he could not help hoping a little himself) and saw nothing but rain-swept woods, another little parcel of fury exploded inside him. he could hear Ron saying "We thought you know what you were doing!", and he resumed packing with a hard knot in the pit of his stomach.
Harry Potter and the Deathly Hallows
I don't understand "packing with a hard knot in the pit of his stomach". What does it mean?