129 lbs (58 Kilograms) 5'8 male, am I underweight.?

I am currently nineteen years old, about six months away from twenty. I weight 58 kgs or 128 lbs and I'm 5'8. I was wondering if I am underweight, I already checked my Body m index and it indicates that I am of normal body weight. Just before the summer I weighted only 118 lbs, and surprisingly enough I gained ten pounds over the summer. I don't know how this happened. I still look the same in body shape. What could be the reason?