She was fighting alongside Joe as well, didn't you see? Probably she was busy dealing with anothe foe at that time
I did see her and she wasn't fighting anyone, it was just her and Joel. That part where the Hunter kicks the door against Joel is scripted.
She was likely still a bit shocked/disheartened to learn that the Fireflies had left. The fact that her and Joel had come all that way, only to have to travel even further, would be pretty draining emotionally. Also, running into the Cannibal scavenging party was kind of a surprise for the two of them.