Why did I get a hernia even though I'm not overweight?

Congenital. Most hernias are the results of weakened connective tissue, which is hereditary but non dominant. There is no asociation between bmi and hernia presentation.
Not uncomon. A hernia is a hole in the abdominal wall through which the inner lining of the abdominal cavity stretches to create a sac. Most of these defects develop in fetal life and are not preventable, regardless of a person's weight. However, being obese may accelerate the growth of these hernias and lead to higher recurrence rates.