I second the celiac disease suggestion. I used to have fainting spells randomly, even while just sitting in a chair. I went gluten free and felt better, But it turned out I had additional food intolerances to sort out. When I went off soy I started getting much better and stopped fainting.

Of course if you aren't eating enough then you are going to have problems also. Another thing that can cause this is hypoglcycemia, (low blood sugar).

IF it keeps happening you should see a doctor and think about anything you have changed in your diet, such as new drugs or vitamins or foods.