Subscapularis Tendinopathy or Strain: This condition is likely given the location of pain (medial aspect of the axilla) which is consistent with the area where the subscapularis tendon resides. The description of the pain as resembling a toothache and the limited changes to range of motion also support this diagnosis, as tendinopathies can cause deep, aching pains and may not significantly limit movement, especially in the early stages.

Adhesive Capsulitis (Frozen Shoulder): Although the patient reports limited changes to range of motion, early stages of adhesive capsulitis could present with pain and minimal restriction in movement. The pain description could fit, but typically, frozen shoulder progresses to more significant limitations in range of motion.
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 could cause pain in the shoulder and axilla, especially if the compression affects the lower trunk of the brachial plexus, which runs close to the subscapularis region.
Bicipital Tendinopathy: While the pain is more medially located, bicipital tendinopathy can sometimes refer pain to the axillary region. However, it's less likely given the specific location of pain described.

Pancoast Tumor: A type of lung cancer located at the top of the lung that can invade the brachial plexus and cause shoulder and arm pain. It's crucial to consider this, especially with persistent pain and any additional symptoms like weight loss or cough.
Brachial Plexus Injury or Tumor: Any condition affecting the brachial plexus can cause pain in the shoulder and axillary region. Given the potential for serious underlying conditions, it's essential to thoroughly evaluate for these possibilities.
Referred Pain from Cardiac or Abdominal Sources: Although less common, pain from cardiac (like myocardial infarction) or abdominal (like gallbladder disease) sources can sometimes be referred to the shoulder region. The description of pain as a toothache could potentially fit with atypical presentations of these conditions.

Quadrilateral Space Syndrome: This involves compression of the axillary nerve and/or posterior circumflex humeral artery as they pass through the quadrilateral space. It's a rare condition that could cause shoulder pain but is less likely given the specific location and description of pain.
Scapulothoracic Bursitis: Inflammation of the bursae located between the scapula and thoracic wall can cause pain in the shoulder region, including the axilla. However, it's less common and might not perfectly fit the described pain location and characteristics.