What is causing hip pain after a fall?

The injury. You could of strained a muscle, or ligament. You could of fractured bone, you could of bruised bone or soft tissue. You could of caused nerve irritation. After the fall, ice and an antiinflammatory are a good idea, if after 3 days there is still pain and discomfort a trip to the orthopedist is a good idea.
Hip pain after fall. If unable to ambulate, consider asap trip to the er to be evaluated with xray and exam for a hip fracture, which may require orif/surgery. Other causes include contusion, hematoma, strain, sprain, bursitis, and nerve root irritation, possible spine compression fracture.