Is this considered underweight?

I'm 18, 5'5" and I weigh 103 lbs. I just went to the doctor, and the nurse asked me if I do a lot of exercise but didn't make too much of a big deal out of it. My primary, however, gave me a look and asked me a few questions and told me that I need to gain. Is me being 103 pounds that bad?
Is this considered underweight?
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