Even though you've tested negative for anaemia, low iron stores may still be the cause of your tiredness. Researchers at Cornell University gave 41 young women who had low iron stores - but not anaemia - a placebo or iron tablets and instructions to exercise.
After 6 weeks, those with the greatest iron deficiency improved their energy level only when they took iron supplements. But supplements can cause serious problems in some people, so consult your doctor before taking any. Or increase your iron intake by eating iron-rich foods, like red meat, legumes and iron-fortified cereals.