a lot of things can cause a stress fracture. I had one in 2000 and i think it was from poor diet (not very nutritious, poor student diet) and being overweight. I take a calcium supplement and eat a very well rounded diet and do not eat too little. Not eating enough will surely cause issues with your bones and lead to a stress fracture. This is why i think you need a structured plan, because according to you, you have issues eating enough calories.