She lost her equilibrium.

She seemed rushing through a blurred green and black aisle of the forest with a gale in her face.

Then, with a sharp jolt, a break, Calico plunged to the sand.

Carley felt herself propelled forward out of the saddle into the air, and down to strike with a sliding, stunning force that ended in sudden dark oblivion.
Upon recovering consciousness she first felt a sensation of oppression in her chest and a dull numbness of her whole body.

When she opened her eyes she saw Glenn bending over her, holding her head on his knee.