Any idea which one it is?
If you have symptoms such as fatigue, exhaustion, shortness of breath, fainting, pale skin, abnormal heartbeat, cough, trouble sleeping, muscle weakness or mood changing, you might have iron deficiency.
Did you know that iron deficiency is the most common known form of nutritional deficiency? After giving birth to four kids and still breastfeeding the last one, I thought my fatigue was completely normal. Until I couldn't get rid of the persisting cough and visited my doctor. He prescribed a test to see if I could be iron deficient and voila, the test was positive! I wasn't aware that women of childbearing age require additional iron to compensate for menstrual blood loss (an average of 0.3-0.5 mg daily during the childbearing years)! 
On a Vegetarian Diet?
Especially if you follow a vegetarian or vegan diet or if you just gave birth or are breastfeeding, you should pay attention to your symptoms. Your body is smart and with symptoms such as fatigue or cough, it is trying to tell you something is off. Although iron can be found in plant foods such as spinach and beans, heme iron, which is found only in meat, poultry, and fish, is two to three times more absorbable than non-heme iron, which is found in plant-based foods and iron-fortified foods.
Also, the bio availability of non-heme iron is strongly affected by the kind of other foods ingested at the same meal. For example, foods that contain chemical compounds like polyphenols, phytates, or calcium make it harder for the body to absorb and store iron. These can be found in such foods as tea, coffee, whole grains, legumes, nuts, seeds and dairy products. 
How To Boost Iron Absorption
Foods containing vitamin C enhance non-heme iron absorption, so if you are a vegetarian or vegan, spinach with lemon juice will be your best friend. Also, add some oranges, tomatoes, berries, kiwi fruit, blackcurrants, broccoli, parsley and capsicum to your iron-rich foods. Avoid drinking tea, coffee or calcium-rich beverages during or directly after consuming a source of iron. You might want to use cast iron skillets while cooking or buy a supplement like this one.
Let me know - what are your favorite recipes to combine iron-rich foods with vitamin C rich foods?