I had this happen too, during a long fast for detoxing reasons. This was after +/- 28 days, i.e. when my body mass index had gone below 16. I then did 1 more week, during which my weight stayed almost the same.
Mind you, my boyfriend, who has always been very skinny, once did a 10 day Water Fast and only lost 2 pounds during the whole 10 days.