You're normal weight (in the healthy weight range) for your height, so of course your body is rejecting the idea of you dropping further; underweight is not a natural state for the body to be in. Runway models don't develop eating disorders for nothing and 500 calories is well on the way to a serious problem. Bear in mind, in order to function at peak efficiency your brain needs a certain amount of energy; without it you may feel depressed, lethargic, irrational, overly-emotional, and your hormone levels may go crazy (that's why many women on extremely restrictive diets like 500 calories stop having their periods; their bodies cannot sustain a pregnancy because they're starving themselves to death and their bodies couldn’t spare the energy, vitamins, or minerals a fetus would need to survive).
Be careful, it's a slippery slope.
Maybe when you start to feel this way you should look at pictures of where you are now, versus where you were when you started your weight loss.