Fibromyalgia: Given the generalized tenderness, normal blood investigations, and absence of specific findings on imaging, fibromyalgia is a strong consideration. It's a condition characterized by widespread musculoskeletal pain accompanied by fatigue, sleep, memory, and mood issues. The tender points along the ribs and the generalized tenderness in the epigastric and suprapubic regions could be consistent with this diagnosis.

Costochondritis: This condition, also known as Tietze's syndrome, involves inflammation of the cartilage that connects the ribs to the breastbone. The tenderness from the 3rd to 11th rib could suggest costochondritis, especially if the pain is reproducible with palpation.
Irritable Bowel Syndrome (IBS): Given the abdominal pain and the absence of alarming features or abnormal findings on USG abdomen, IBS could be a consideration. It's a gastrointestinal disorder characterized by chronic abdominal pain, bloating, and alteration of bowel habits in the absence of any mechanical, inflammatory, or biochemical abnormalities.
Stress or Anxiety-related Disorders: These can manifest with somatic complaints including chest and abdominal pain, especially in adolescents. The normal investigations and the nature of the pain (on and off for 2 months) might suggest a psychological component.

Pulmonary Embolism: Although less common in young individuals without risk factors, pulmonary embolism can present with chest pain and should be considered, especially if there's any history of recent immobilization, family history of clotting disorders, or use of oral contraceptives.
Cardiac Conditions (e.g., Pericarditis, Myocarditis): These can cause chest pain and, although rare in healthy adolescents, are critical to diagnose early. The absence of specific ECG findings or elevated cardiac enzymes does not entirely rule out these conditions.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can present with abdominal pain and might not always have abnormal findings on initial USG or blood tests. A high index of suspicion is needed, especially if there are other symptoms like weight loss, diarrhea, or blood in stool.

Sickle Cell Crisis: If the patient has sickle cell disease, a crisis could cause both chest and abdominal pain due to vaso-occlusive episodes. This would be less likely if the patient does not have a known history of sickle cell disease.
Ehlers-Danlos Syndrome: This is a group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues. Although rare, it could potentially cause widespread pain and joint hypermobility.
Mitochondrial Myopathies: These are a group of disorders caused by dysfunctional mitochondria, which could lead to muscle pain, including chest and abdominal wall muscles. However, these conditions are rare and typically present with other systemic symptoms.