17yr girl athletic, ferritin<10, not anaemic, oral iron for 18mths, memory loss, no coeliac, light periods, extreme tiredness, joint pain. Any ideas?

Go to see your MD. You have iron deficiency- despites taking the oral iron. How do you take it? If you only take once a day- then increase to 3x/day. Take it on empty stomach and drink it with orange juice or vitamin c to increase absorption. How is your diet? Increase intake of food that rich in iron like spinach, beets, liver etc. See-www.Ironrichfood.Org. If oral iron doesn't not help- then IV iron is needed.
Overdoing sports. Stop your athletic involvement for a while and your symptoms will begin to improve.