This is just my opinion:
Your BMI is 18.. So you are not underweight according to that. I am one of those persons who don't believe in BMI anyway, because my BMI was always around 18 or less on any diet and I was always eating a lot. This is because of genes, constitution, fast metabolism etc. And I don't understand what's about bone loss?
Look in the mirror.
If you don't like what you see, then you have a problem.
Anyway, this may be helpful: http://www.nealhendrickson.com/mcdougall/030700puhowdoIgainweight.htm
As you see on charts you have a good weight.