What could cause severe hip pain?

Many things. Here is the list - osteoarthritis, gout, strain, labral tear, inguinal hernia, sports hernia, avascular necrosis, bone contusion, tendonitis/bursitis, pinched nerve in lower back, etc. As you can see, the list is pretty long. Remember, common things are common, so things like arthritis, strains/sprains and tendonitis tend to top the list. Lastly, see your physician if the pain persists.