Pain in the shoulder and upper back can be caused by many different problems. A bulging disc in the upper spine is one possibility. If the disc is pushing on a nerve, it would affect the power of all the muscles that nerve serves and cause weakness, and it would affect all of the sensory area that that nerve serves. Shoulder and upper back pain can also be caused by pain in the shoulder joint due to injury, or arthritis. Pulling a muscle could also cause pain in that region.
Could you describe to me when the pain started? Is it intense, worse when you move or at rest. Describe the type of pain and exactly where it is. Is there any numbness or tingling? Does advil, tylenol or anything help?

That could be a disc compressing a nerve. that can cause shooting pains, numbness, and weakness. another test that can help confirm nerve compression is a nerve conduction study/electromyogram (EMG). It looks at the function of the nerves that serve your arm. If there is a disc compression of the nerve, you should see a neurologist, and possibly a neurosurgeon.

An MRI is a picture of the anatomy. IT looks at what structures are next to each other, and whether or not everything is in the right place. THe EMG/nerve conduction looks at the function of the nerves and muscles. IT can give an idea of which nerves/muscles are not functioning properly, and link it with the disc bulge. (to make sure that the the disc bulge really is the cause).

The first step would be to go to a neurologist to examine you. Based on that exam, the neurologist would then decide on the next step, EMG/Nerve conduction study(NCS) and/or MRI. Usually with EMG/NCS, a sample of nerves is chosen by the neurologist (based on your exam) from the affected extremity (arm, in this case) plus an additional normal extremity (opposite arm or one of the legs) for comparison.