Thank you for the query.
Diarrhea and constipation
alternately indicate last part of small intestine
or large intestine issue. Most probable issue at your age is Crohns disease
. This disease is an autoimmune issue. Main symptoms of it are diarrhea/constipation, blood in stool
, mucus in stool, weight loss, bloating, nausea. Blood is usually mixed with the stool.
Similar symptoms can appear due to intestines infection. But Crohns should be ruled out at first.
I suggest you to do not wait anymore and force your doctor to do either colonoscopy
or camera swallow. Abdominal CT with oral contrast (enteroclysis) may be necessary as well.
Hope this will help.