Is it bad to be underweight if balanced diet?

In general "no" In general the" thinner the better" in reference to increasing one's longevity. The genetic implications for weight..Too thin or too fat can be as high as 77%. Make sure that your thyroid function has been evaluated since hyperthyroid states can cause unhealthy weight loss there are many co-morbitities associated with being overweight.
How do you feel? My colleagues have given you good advice. If you can stay competitive in your favorite sport, and you feel well, your weight is fine. You're a person, not a beancounter's average -- "underweight people dying sooner" doesn't control for drug addicts, alcoholics or cancer victims. Many elite track-and-field athletes are "underweight". Eat sensibly and be thankful for good health.