Being thin may be normal for you, but you could have a malabsorption syndrome like Celiac disease. A doctor could check for you.
I was underweight in school. I remember I weighed 44 lb in 4th grade, 50 something in 5th grade, 60 something in 6th grade, and 80 something in 8th grade. That translate to the range you are talking about. I am 5'6", so I'm not short, either. And I turned out to have Celiac disease, but not detected until much later.
You can try having a liquid snack such as a green smoothie to see if that helps. Try chewing your food better. Eat more hamburger than steak. Do any foods cause migraines? You could have food allergies.
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