Fibromyalgia: This condition is characterized by widespread muscle pain, stiffness, and chronic fatigue, which aligns with the patient's symptoms. The presence of drenching night sweats could be related to sleep disturbances often seen in fibromyalgia.

Rheumatoid Arthritis (RA): Morning stiffness and muscle pain are common in RA. While night sweats can occur, they are less typical. However, the chronic fatigue, both physical and mental, could be attributed to the inflammatory nature of RA.
Hypothyroidism: This condition can cause fatigue, muscle pain, and stiffness. Some patients with hypothyroidism may experience night sweats, although it's not a primary symptom.
Chronic Fatigue Syndrome (CFS)/Myalgic Encephalomyelitis (ME): Characterized by profound fatigue that is not improved by rest, CFS/ME could explain the patient's chronic fatigue. Muscle pain and night sweats can also be part of the symptomatology.

Lymphoma: Although less common, lymphoma can present with night sweats, fatigue, and muscle pain. It's crucial to consider this diagnosis due to its potential severity and the need for early intervention.
Tuberculosis (TB): TB can cause night sweats, fatigue, and muscle pain. Given its infectious nature and the importance of early treatment, TB should not be overlooked.
Multiple Myeloma: This plasma cell disorder can lead to fatigue, bone pain (which might be perceived as muscle pain), and night sweats due to the production of abnormal proteins.

Sjögren's Syndrome: An autoimmune disorder that can cause fatigue, muscle pain, and potentially night sweats due to systemic inflammation.
Polymyalgia Rheumatica (PMR): Characterized by muscle stiffness and pain, particularly in the morning, PMR could be considered, although night sweats are less commonly a primary complaint.
Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodic symptoms including sweating, fatigue, and pain, although the pattern of symptoms might differ from what's described.