I have a 2cm hiatal hernia i am 24 years old and underweight, i have symptoms since maybe 3 years. how can i know if its congenital or acquired?

Probably acquired: Since you are having symptoms for 3 years ,it is most likely acquired . It may happen due to increased abdominal pressure , strainig during passing stool or another cause . Many times the cause is not known.

Why does it matter?: If u have chronic reflux (GERD) symptoms as a result, then u should be treated to prevent progression or serious complications (ie, erosive esophagitis, ulcers, barretts, etc). Treatment includes medications (H2blockers/PPIs) and lifestyle changes. Differentiating congenital vs acquired-traumatic vs atraumatic (sliding vs paraesophageal) is important only when considering surgical management.

See below: Most congenital hiatal hernias are diagnosed before or shortly after birth and frequently present dramatically. There are some that do not present until adulthood. The actual location in the diaphragm is sometimes another way to help differentiate. Discuss with your doctor for further details.