Hello! Thank you for the query. Diarrhea and constipation present alternately can be very serious symptoms. Sometimes partial bowels obstruction appears in such way. At your age, it is possible that Crohns disease can give such symptoms. Other possible reasons of your symptoms are chronic bacterial infection, thyroid disease, bad diet. As it lasts some time now, I suggest you to visit your doctor and have some diagnostics. Stool tests and stool culture tests should be done. Blood work, liver tests, amylase, urine analysis, abdominal ultrasound and abdominal X-ray should be also done. If all this tests will be negative, colonoscopy and abdominal CT with oral contrast (enteroclysis) should be the next step. Hope this will help. Regards.