You could be allergic to soy. But I think it's more likely you're just not getting enough calories per day. Eating less than 1,000 isn't a good idea, because it's borderline-starvation (900 or less per day is classified as starvation), which only encourages your body to store fat any time you eat a little more than that, because it goes in starvation-mode. So you should probably try to get somewhere between 1,200 and 1,500 per day, depending on your body type and all that. Also, how frequently are you eating? If you only eat 2 or 3 meals a day, with no snacks, then it could just be a case of low blood-sugar. The only other thing I can say about this, is make sure you're drinking enough water every day, because dehydration symptoms are similar to those too.