What might cause pain in the thigh that seems to move around in buttocks and groin if the doctor suggested an ultrasound and venogram?

Hip Joint and spine: The pain in the anatomical region you have mentioned could be from the hip joint, spine, and soft tissue infection. Venous blood clots can result in pain and swelling in the calf and thigh but not in the buttocks. A good history and physical exam will usually identify the system involved.

Hip, back, etc.: Several things cause pain in those areas: low back problems with irritation of nerve (radiculopathy), groin, obturator hernia, hip degeneration, pelvic disease with irritation of nerves. Evaluation with ultrasound may help identify the problem, but best to start with physical examination.