Constantly tired. Getting me down,help!?

I don't have any real idea of what it could be. I have been on iron tablets in the past but last time i was checked,my blood levels were low but not dangerously so. I have no energy for anything,find myself breathless at the slightest bit of exertion and even on a night out,i am desperate to go home to my bed at half eleven so i cant enjoy a night out anymore! Im 26,usually healthy,eat pretty well,exercise three times a week,I am not overweight in the slightest but i still feel like i could sleep my life away. I fal asleep at the drop of a hat,constantly feel like shutting my eyes over. I get plenty sleep but never feels enough! Its got to the stage im getting really pissed off all the time. I have had depression for a while and was on anti depressants too,but right now my emotions are severly messed up. constantly crying,constantly anxious and worried. Could my tiredness be linked to depression? It has only got worse recently in the past month.,