I havemild pain in my shoulder, but only during certain motions, like extending my arm out palm up and hitting a volleyball. do i need a contrast mri?

Not Yet.: It seems you have bursitis of your shoulder. Before you jump the gun to getting an mri, first try to see a doctor and start using some anti-inflammatory medications (i.E: ibuprofen) and some physical therapy. If no improvements in few weeks , then an MRI may be warranted.

Slow down: Going to an MRI may not be necessary. Have you been to your doctor and/or an orthopedic surgeon? If not, i don't think you need one at this point.

Need exam first: You should have your doctor perform a physical exam first. That may determine the cause of the problem without further testing. If an MRI is needed, shoulder MRI to look for a rotator cuff tear or muscle injury is typically done without contrast. The physical exam will help guide what type of additional testing, if any, is indicated.