Carol does bump into the arm mostly because she didn't expect for him to throw it out in front of her. But the fact that he put it out and is now pulling out his gun tells her that something is about to go down.

So she moves from a relaxed walking position into one more suitable for throwing a punch and starts building up the energy under the surface of the skin on her hands. Carol knew better than to ask him to clarify as that would give potential attackers a clearer target. So instead she goes back to scanning hiding areas.