Musculoskeletal Injury (Costochondritis or Rib Fracture): This is the most likely diagnosis given the location and nature of the pain. A direct blow to the chest or a fall could cause a fracture or strain the muscles and cartilages attached to the ribs, leading to pain.

Intercostal Muscle Strain: The intercostal muscles between the ribs can become strained due to overuse, injury, or severe coughing, leading to pain in the chest wall.
Precordial Catch Syndrome: This condition involves sharp, stabbing pain in the chest that can be caused by inflammation of the cartilage that connects the ribs to the breastbone. It's more common in adolescents and young adults.
Shingles (Herpes Zoster): Before the rash appears, shingles can cause pain in a specific dermatomal distribution, which could include the area around the 6th rib.

Pulmonary Embolism: Although less likely, pulmonary embolism can cause sudden onset of chest pain and is a life-threatening condition that requires immediate medical attention.
Myocardial Infarction (Heart Attack): Chest pain, especially if it radiates to the arm, jaw, or neck, and is accompanied by shortness of breath, sweating, or nausea, could indicate a heart attack.
Pneumothorax: A collapsed lung can cause sudden, severe chest pain and shortness of breath. It's a medical emergency.

Tietze's Syndrome: A rare inflammatory condition that affects the cartilages and surrounding tissues of the ribs, causing chest pain and swelling.
Sickle Cell Crisis: In individuals with sickle cell disease, a crisis can cause severe pain episodes, including chest pain, due to vaso-occlusion.
Osteomyelitis or Osteitis of the Rib: Infection of the bone or bone marrow can cause pain, fever, and swelling over the affected area, which is a rare but serious condition.