possible lack of oxygen to brain

My grandmother was told by a dr. that she might have a lack of O2 supply to brain based on the fact that she has constant pain on the back of her head. But I am worried that this could be a brain tumor.  No tests have been done yet. I would like to ask you if you think the dr. might be correct and what type of test would more appropiate to be able to find the actual cause of her pain.