Subacromial Bursitis: This condition is characterized by inflammation of the bursa that cushions the rotator cuff tendons, leading to pain and swelling in the shoulder region, especially underneath the collar bone. The patient's symptoms of pain, soreness, swelling, and difficulty lifting the arm are consistent with this diagnosis. The onset of symptoms without a specific incident like a fall could suggest an inflammatory or overuse etiology.

Rotator Cuff Tendinitis: Inflammation of the tendons of the rotator cuff muscles can cause pain and weakness in the shoulder, especially when lifting the arm. This condition could explain the patient's symptoms and is a common cause of shoulder pain in adults.
Adhesive Capsulitis (Frozen Shoulder): Although typically presenting with gradual onset of stiffness and pain, early stages of frozen shoulder could manifest with pain underneath the collar bone and limited mobility, which might fit the patient's description of difficulty lifting her arm.
Strain or Minor Trauma: Despite the patient's denial of any fall or incident, minor strains or unnoticed trauma could lead to inflammation and pain in the shoulder region, including underneath the collar bone.

Thoracic Outlet Syndrome: This condition involves compression of the nerves and/or blood vessels that pass into the thoracic outlet (the space between your collarbone and first rib). It can cause pain in the shoulder and arm, and while less common, missing this diagnosis could lead to significant morbidity if not addressed.
Pulmonary Embolism: Although the primary complaint is localized to the shoulder, it's crucial to consider that pain underneath the collar bone could be referred pain from a pulmonary embolism, especially if the patient has risk factors for deep vein thrombosis or embolism.
Myocardial Infarction (MI): Referred pain from an MI can sometimes present as shoulder or arm pain. Given the potential severity of an MI, it's essential to consider this in the differential diagnosis, especially in a patient of this age group.

Pancoast Tumor: A rare type of lung cancer that grows in the upper part of the lung and can cause shoulder pain due to local invasion of the brachial plexus or ribs. This would be an uncommon cause of the patient's symptoms but is worth considering in a differential diagnosis due to its severe implications.
Osteonecrosis of the Humeral Head: A condition where the blood supply to the bone tissue at the head of the humerus is interrupted, leading to bone death. This could cause shoulder pain but is less common and might not directly explain the pain's location underneath the collar bone without other symptoms.