be SURE to have your thyroid checked! hyperthyroidism can cause rapid weight loss like this because it affects your metabolism. your doctor should test you for this along with the other routine blood work that is also important.
If you are not trying to loss weight by decreasing food intake and not exercising, then it has to be something medically. You really do need to be checked by your MD and have him do blood-work to see why you are losing weight.