There may be some clues on your blood film ( taking a sample of your blood and looking under the microscope) that you may have iron deficiency, even though your hematocrit is within normal, such as microcytosis ( which means small red blood cells or low MCV),
I recommend that you repeat the ferritin test to confirm the result, along with serum iron, and a blood film ( or smear), and a Hematocrit/Hemoglbin level and platetets count.
If iron deficiency is confirmed based on the above tests, then the cause of it should be investigated.
Usually the cause are losses from bleeding from the gastrointestinal tract ( sometimes might be not apparent because of its small quantity), or other sources of bleeding such as blood in urine, of course in women severe bleeding during their menses may sometimes cause iron deficiency, other causes include intestinal malabsorption, and inadequate iron intake if your diet does not contain enough iron.
Once the cause of iron deficiency is known, appropriate treatment according to the discovered cause would be given, and you will also need to take iron supplements for a certain amount of time to replenish your iron stores in your body, regardless of the cause.