I am also severely underweight.

• It's not anorexia because I don't try to consciously lose weight.
• It's not about eating, I eat a lot of chocolate and fast food.

If you think we have a similar problem, in my case it's because of overly active thyroid gland. I've seen doctors about it, there are possible options like medicine or removal of thyroid but then you have to take hormones because a small amount is required.

However they said that I'm not unhealthy so the best option is to do nothing.

• If you're female enjoy your model body?
• If you're male, I found that with workout I can build nice toned muscles, but it must be done every day or every second day, otherwise the muscles disappear! Don't lift too much weight, I found dumbbells and pushups, or swimming are the most effective.

I'm taller than you and weight a bit more, maybe your case is more severe, so I guess ask a doctor, but have some doubts, don't blindly go for every possible treatment...