I was in an auto accident in sept. And I have pain in my left shoulder. The doctors have x-rayed and don't see anything wrong, but it really hurts. What should I do?

See a specialist. There are many injuries that don't show up on an xray. It is time to seek the evaluation of an orthopaedic surgeon. If necessary he will order an MRI to assist his evaluation.
X-rays not see all. The shoulder is a very complex joint. There are muscles, tendons, bursas, nerves that are wrapped together in a very efficient fashion. X-rays don't see everything and you need further evaluation to look at all of these other components. You need to discuss this with your physician and you may need referral for further examination, testing, maybe more imaging with ultrasound or mri.
See orthopedist. Shoulder pain can be referred from the neck or it could be locally from the shoulder. Common local problems include but not limited to spasms, arthritis in the ball and socket or ac joint, bursitis, impingement, rotator cuff re-tear, labral tears, biceps problems, stiffness, and instability. See an orthopedist to diagnose the problem an MRI with dye in the joint may be very helpful.
Rotator cuff/labrum. The majority of shoulder problems are related to soft tissue problems. The rotator cuff tendons and the labrum (cartilage) are frequently injured. Time to see an orthopedic doc for a thorough exam and possible mri.
Shoulder evaluation. Usually by 4 months minor problems will correct unless there is muscle imbalance continuing to impair proper shoulder motion. It it has not responded by now, it would be worth seeing a shoulder specialist.