I'm 19 and have lost hair at the bottom of my head. it is about 3 cm in diameter and whilst hair is growing back sparsly, there is a smooth bald spot?

Alopecia areata: You might have alopecia areata. Best to see your Family Doc for referral to Dermatologist for evaluation & treatment. In meantime, check out https://www.aad.org/dermatology-a-to-z/diseases-and-treatments/a---d/alopecia-areata, http://www.webmd.com/skin-problems-and-treatments/hair-loss/tc/alopecia-areata-topic-overview & https://www.naaf.org/.