Any type of gastroenterities - which should be suspected given the travel history - can lead to blood in the stool. Other causes include an anal fissure or hemorrhoids. Being only 19 years of age, things like polyps and cancer are unlikely.
I would suggest testing the stool for parasites, as well as giardia and other bacteria. If positive, then appropriate treatment can be made. If the bleeding continues, a flexible sigmoidoscopy can be considered to further evaluate the cause.
I would consider a gastroenterology referral.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Thanks,
Kevin, M.D.