What can cause terrible hip pain that moves down to the groin?

Hip pain. Evaluate the patient for a psoas (hip flexor) abscess; a direct, indirect or sports hernia; image the hip and rule out underlying arthritis. Carefully examine the lower back and in a female ensure that there is no uterine fibroid, ovarian cyst or pid.
Tough one. Hernia, sports hernia (athletic pubalgia), labrum tear, hip impingement, hip flexor tendonitis, adductor muscle strain, hip arthritis, stress fracture, infection, tumor, and a many others. Best to see an orthopaedic surgeon for an exam and an x-ray.