I think "plumdumplings" has given the most accurate answer. If you are a kid, under 14, that's fine. As an older teen, or young adult your weight is on the very low end, and still can be healthy, but best to ask your doctor. If you are an adult it's clearly underweight.
In any case - if you are restricting, talk to someone about this Because it can get worse and out of control. Don't wait.