Based on details you have bilateral headaches with MRI showing pituitary tumor 11mm (when size is more than 10mm it becomes macroadenoma).
This tumor can cause local compression and causes hypersecretion of hormones.Hence it is advisable to get check your vision, visual field, prolactin and other hormone levels.Derangement of this itself produce headache.
In pituitary tumors
, vascular headache
occurs, treatment of choice is cyproheptadine
Arnold chiari malformation
cannot produce headache unless obsrtuctive hydrocephalus occurs