It can happen when someone becomes anemic. This can be because of iron deficiency. If that is the reason, then start eating more beans and eat the following foods to help absorption.
Fruits: Orange, Orange Juice, cantaloupe, strawberries, grapefruit etc
Vegetables: Broccoli, brussels sprouts, tomato, tomato juice, potato, green & red peppers
You can't just stop eating meat and fish all of a sudden after your body is used to absorbing those nutrients everyday and expect to get the same nutrients from eating vegetables. You have to account for all the missing nutrients. You can still get them with a veggie diet, but you have to be more careful about how much you are consuming and you need to make sure you are adding some protein and iron and other essential vitamins in there.
Google:
Iron Rich Foods
Not everybody's bodies are the same. We don't all have the same metabolisms and we don't all absorb nutrients equally. We wouldn't have diabetic people if everybody required the same things. Your body may need more iron than my body because it absorbs differently.