What Causes Head Pain With Swelling And Pressure?

I have been experiencing terrible pain in the side of my head, it feels swollen or bigger. I am seeing a neurologist, he put me on neurontin 300mg tid. I went to get a mri last week of the neck and head. Needless to say when they went to do the head I had to stop because of the pain and pressure in my head. So I am going back next week . They are going to give me anethesia. any ideas what this could be?