Hmmmm....
That's odd!
I'm just the opposite -- if I eat just a little extra -- such as a little more tofu, or even too much raw salad with vinegar -- my weight jumps 2 pounds or so.
Perhaps you have an unusually fast metabolism -- or, as I suspect, your body is being inefficient in digesting food into calories. That's the opposite of
what usually happens on a CRON diet.
However, you're not alone -- at least one distinguished, universally respected member of the Cr Society -- Michael Rae -- was also very skinny before starting
CR, and currently weighs in the teens -- but his BMI, although enviable, isn't as low as yours.
The downside: It's POSSIBLE that something is physically wrong -- one very unlikely possibility: roundworms or other parasites.
Unlikely as that is, it might be a good idea to see a competent GP to rule out something like this.
Anyway, welcome!
Someone will know more than I do!
(Many).
-- Saul