Feels like muscle spasms on left side of head, happened once or twice before, it's lasted all day on/off. been very tired. temporal arteritis?

You're too young: Temporal arteritis mainly affects people over 70 as they get the diseases that cause it. Young people don't. Unlikely under 50. It causes arteries to become blocked & can lead to vision loss & stroke. Diagnosis is made by blood tests & a biopsy of the temporal artery. Symptoms: headache, scalp tenderness (when brushing hair or touching their temples) , pain with chewing, fever & muscle weakness.

Nope: Can't say what you do have, but you definitely do not have temporal arteritis, as it never affects 18 year olds.

Jaw and Muscle Pain: It appears that you have a serious TMJ and muscle dysfunction that causes these muscles to go into a spasm and the jaw joint (TMJ) to be misaligned. You seriously need treatment from a dentist that is very skilled at treating tmj. Only about 5% of dentist are really good in this realm in that they are very familiar with multiple treatment options to select from based on the patient needs.