This might be a stupid answer, but I'd say it's because you're malnourished. That's why it happens in Ethiopian children. Do you take a multivitamin? I know how it is to restrict, I'm still recovering, but if you can you should try to get in as many nutrients as possible.

Vitamins are usually calorie-free, but are best digested with other food. So try to take them when you're ingesting something else, too, for optimum absorption. Also, some vitamins don't digest as well as others -- you can test this by putting your vitamin in water for a few hours and seeing how well it dissolves. Some won't dissolve for days, which means that even in the acidic environment of your gastrointestinal system, it's unlikely to be absorbed properly.

Sorry for the really long answer to such a short question.