Hello, i'm 19 years old, have had very occasional red blood in stool since last summer. now some change in stool caliber. parents say not to worry?

Sounds like you are : Constipated . Occasional blood in the stools are normal if you suffer from constipation.Change in caliber might be more concerning . You need to discuss this with your physician to be sure it is just constipation and not colon cancer or other medical conditions causing blood and change in stool caliber . Erring on the safe side. Please get checked.

Maybe not: Someone with your age most likely nothing, however, it would be easy to go see either a general surgeon or a colorectal surgeon. Could be a hemorrhoid but i would get it checked out. You are considered an adult at 19. If you feel more comfortable, call your primary doctor.