I should mention that no matter how much or how little I eat, my weight is constant at 118 lbs. I think that is weird. Maybe the body makes an adjustment that is not apparent. Perhaps the same is true of you.
Is your family genetically lightweight? My family is, even though we eat normally (and the slimmest of us eats the most). In our case it has nothing to do with the thyroid.
Have you been losing weight, or have you always been thin? If the low weight is recent then you should see a doctor.
It might have something to do with your thyroid. You might have hyperthyroidism. Im not a doctor though.
Maybe it's because u have a fast metabolism.. My boyfriend is 6 ft 5 and a stick and he eats like a horse I even tried him on a high carb and fatty foods diet with no excercise other than his daily moving around and he didn't put on anything... He has a fast metabolism and has even tried weight gain supplements but none help.. My suggestion would be to consult a doctor or dietition.. Mayb also get ur thyroid looked at.. Goodluck