Osteoarthritis (OA) of the shoulder: Given the patient's history of arthritis and the nature of the pain (worsening with activity and at night), OA is a highly plausible diagnosis. The degenerative changes associated with OA can lead to pain and stiffness, especially in a joint that has been previously affected by arthritis.

Rotator Cuff Tendinitis: This condition is common in individuals with a history of shoulder arthritis and can cause pain that worsens with activity and at night. The rotator cuff tendons can become inflamed due to repetitive strain or direct pressure, leading to pain and limited mobility.
Adhesive Capsulitis (Frozen Shoulder): Although more commonly associated with a gradual loss of motion, adhesive capsulitis can also present with pain, especially at night, and can be related to or exacerbated by underlying arthritis.
Bursitis: Inflammation of the bursae around the shoulder joint can cause pain, especially with movement and at night. It can be associated with arthritis or result from repetitive activities.

Septic Arthritis: Although less likely, septic arthritis is a medical emergency that requires prompt diagnosis and treatment. It can present with severe pain, especially at night, and is associated with fever, redness, and swelling. A history of arthritis might increase the risk of septic arthritis due to potential joint damage.
Fracture: A fracture, particularly of the humerus or scapula, could present with worsening pain at night and with activity. It's crucial to consider this diagnosis, especially if there's a history of trauma or osteoporosis.
Referred Pain from the Cervical Spine: Pain from cervical spine disorders (e.g., herniated discs, cervical spondylosis) can refer to the shoulder region. This diagnosis is critical to consider, as the treatment approach would be significantly different.

Polymyalgia Rheumatica (PMR): This inflammatory condition can cause shoulder pain and stiffness, typically worse in the morning and at night. While less common, it's an important consideration, especially in the absence of a clear mechanical cause.
Neoplastic Processes: Tumors, either primary in the bone or soft tissues of the shoulder or metastatic, can cause night pain. Although rare, this diagnosis must be considered, especially if other systemic symptoms are present.
Infectious or Inflammatory Arthropathies: Conditions like rheumatoid arthritis, psoriatic arthritis, or infectious arthritis (e.g., Lyme disease) can cause shoulder pain. These diagnoses are less common but important to consider, especially if there are systemic symptoms or a history suggestive of these conditions.