I've been having headaches, dizzy spells and increased eye pressure. My doctor ordered a brain scan? What may be suspected?

Migraine. Most likely you have migraine. The brain scan is to rule out a tumor or hydrocephalus or vascular malformation.
Usually nothing bad. This description is typical of a sinus problem. You could have a new or chronic sinus infection. It could also be due to excess fluid production in the brain called pseudotumor cerebri or idiopathic intracranial hypertension. Mri is standard in the work up. Brain tumor is least likely- a common fear and misconception.