Obstacle, challenge, or threat.
A moving or rolling boulder might represent momentum, critical mass, overwhelm, or the idea of being out of control.
Consider also the context of the boulder and how you felt about it.
See Also: Stone, Hard Object, Big, Blockage or Obstacle
[1]1. Strength and power.
2. Obstacles.
3. Severe, seemingly insurmountable difficulties are in the offing.
[2]See Also: Rock(s).
[3]