You say you're not anorexic and then that you're severely restricting your calories. You are anorexic. If your bmi is still above 18, then you're not "clinically" anorexic, but your eating habits, assuming they're as consistent as you say, are clearly anorexic. Bear in mind that <900 cal is a starvation diet. If you keep on this way, your bmi will soon become clinically anorexic and head even lower. The girls I know who eat as little as you (and exercise it off) are not only anorexic but have bmis of <15. Feeling better about not eating is a key anorexic thing. The extreme willpower you go to when you restrict makes you feel mentally stronger. Be careful about imagining that you're healthy just because you don't want to have bones showing.
Once you already have the anorexic mentality, you start to find one goal isn't good enough after you've made it. Having your bones show may seem undesirable to you now, but surely you once thought it was abnormal to eat only 150 calories?? Be careful: when you become anorexic, your paradigm/ways of thinking and seeing the world change in little steps at a time, and you can soon find yourself thinking 5 stone as a weight is normal and desirable. Anorexics are seldom satisfied with the weight they've lost. The desire to lose weight stems from outside problems that you can't control: you take all your problems and insecurities and see them represented in fat. And you imagine if you lose weight, you'll have control and things will get better. But they're still there, and you feel compelled to lose more weight.