Subacromial Bursitis or Rotator Cuff Tendinitis: This is a common cause of shoulder pain in young adults, often related to overuse or repetitive strain injuries, even without a specific remembered incident. The pain is typically localized to the shoulder region and may worsen with movement.

Adhesive Capsulitis (Frozen Shoulder): Although more common in older adults, it can occur in younger individuals, especially those with a history of diabetes or after a period of immobilization. It presents with gradual onset of pain and stiffness in the shoulder.
Scapulothoracic Bursitis: Inflammation of the bursae between the scapula and the thoracic wall can cause pain that may be referred to the shoulder area. This condition is often related to poor posture or repetitive movements.
Referred Pain from the Cervical Spine: Cervical spine disorders, such as herniated discs or facet joint syndrome, can refer pain to the shoulder region. This is more likely if there is associated neck pain or stiffness.

Pulmonary Embolism: Although rare, a pulmonary embolism can cause referred pain to the shoulder, especially if the embolism is located peripherally. This would be accompanied by other symptoms such as sudden onset of shortness of breath, chest pain, or cough.
Myocardial Infarction: In women, especially those with risk factors, myocardial infarction can present atypically with pain referred to the shoulder or arm. Other symptoms might include shortness of breath, nausea, or fatigue.
Pancreatitis: Acute pancreatitis can cause severe abdominal pain that may radiate to the back and shoulder. Nausea, vomiting, and abdominal tenderness would also be present.

Thoracic Outlet Syndrome: Compression of the nerves and/or blood vessels that pass into the thoracic outlet (the space between your collarbone and first rib) can cause shoulder and arm pain, among other symptoms.
Osteonecrosis of the Humeral Head: Although more common in older adults or those with specific risk factors (e.g., long-term steroid use), osteonecrosis can cause shoulder pain. It is a rare condition in young adults without predisposing factors.
Infectious or Inflammatory Arthritis: Conditions like septic arthritis or rheumatoid arthritis can cause shoulder pain, although they are less common and usually accompanied by other systemic symptoms or joint involvement.