Im 18 , ive missed 2 periods and im definitely not pregnant cause ive never had sexual intercourse. it could be stress related because i just started university and its a big change. is this normal and when should i see a doctor?

Not unusual and:: While not quite "normal" it's not unusual.Yes,it's likely stress-related and probably not serious.I advise reducing stress: Eat a healthy diet rich in fruit and veggies,adequate sleep and moderate exercise (intense exercise can worsen this).Try a good multivit. and magnesium (as taurate or glycinate).Herbs like Vitex and Ashwagandha may help,as can acupuncture. See a Dr. if not better in 6 months.