What could be causing my alternations between constipation and diarrhea. i am a teen?

Could be IBS: Possible irritable bowel syndrome. See your doctor.

Irritable bowel?: You may have irritable bowel syndrome, or just a diet that could be better. Eat some fiber daily (fruits/veggies), drink more water (decrease caffeinated drinks), add a fiber supplement like benefiber. Regular physical activity helps decrease GI symptoms. Are you stressed out? That may be part of it as well. Make note of any foods that seem to make you swing to one extreme and eat less of it.

Irritable bowel Synd: Your intestinal tract is irritated and hence alternates between diarrhea and constipation. Eliminating dairy and gluten (both of which have high allergenic potential) may offer some relief. Peppermint tea, oil have been shown to help a lot. See your doc for a good history and physical. You may also find that a chinese medicine practitioner can help. Taking enzymes with meals may help too.