Viral Gastroenteritis: This is the most likely diagnosis given the combination of abdominal pains, nausea/vomiting (N/V), constipation, and diarrhea for 7 days. Viral gastroenteritis is common in adolescents and can cause a variety of gastrointestinal symptoms.

Irritable Bowel Syndrome (IBS): IBS could be considered given the mix of constipation and diarrhea, although the acute onset might be less typical. However, it's a common condition in adolescents that can present with variable bowel habits and abdominal pain.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis could present with chronic diarrhea, abdominal pain, and vomiting. The duration of symptoms could suggest a more chronic condition like IBD.
Food Poisoning: Although typically shorter in duration, food poisoning could cause the described symptoms, especially if the patient has been exposed to contaminated food or water.

Appendicitis: Although the symptoms have been present for 7 days, which is longer than typical for appendicitis, it's crucial not to miss this diagnosis due to the risk of perforation and peritonitis. Atypical presentations can occur, especially in younger patients.
Intussusception: This condition, where a part of the intestine telescopes into another, is more common in younger children but can occur in adolescents. It presents with abdominal pain, vomiting, and sometimes currant jelly stool.
Mesenteric Lymphadenitis: This condition can mimic appendicitis and presents with abdominal pain, fever, and vomiting. It's often associated with viral infections.

Giardiasis: An intestinal infection caused by the Giardia parasite, which can lead to chronic diarrhea, abdominal cramps, and weight loss. It's less common but should be considered, especially if the patient has been exposed to contaminated water or has traveled to endemic areas.
Celiac Disease: An autoimmune disorder that causes the immune system to react to gluten, leading to small intestine damage and symptoms like diarrhea, abdominal pain, and malabsorption. It's less likely given the acute presentation but could be considered in the differential for chronic gastrointestinal symptoms.
Abdominal Tuberculosis: Although rare in many parts of the world, abdominal TB can cause chronic abdominal pain, diarrhea, weight loss, and other systemic symptoms. It would be considered based on exposure history and endemic area.