Other than a non cancerous tumour what else could be causing my prolatin levels to be high and have no period for 5 months?

Prolactin: There are various reason for prolactin levels to go up, one of the most common ones is a prolactinoma or benign pituitary tumor. Also breast stimulation, hormonal problems, medication, exercise, stress, among others.There are other more complicated diseases like lupus and multiple myeloma but a less common than the above especially at your age. F/u with your doctor on this to get definite answer.

Good question: I'm going to assume you are taking no medications that could do this. Illness other than prolactinoma, and head trauma, serious enough to cause hyperprolactinemia is usually obvious. With this history, it's highly probably that you do have a prolactinoma, and should go ahead with the imaging and even consider empirical treatment if one's not visible.