De Quervain's Tenosynovitis: This condition, which affects the tendons on the thumb side of the wrist, is a common cause of thumb pain, especially with activities that involve grasping or twisting. The absence of swelling, bruising, deformity, or crepitus, along with decreased flexion due to pain and metacarpal tenderness, supports this diagnosis.

Trigger Thumb: Characterized by pain and stiffness in the thumb, trigger thumb can cause decreased flexion and may present with tenderness over the metacarpal area. It's less likely than De Quervain's given the specific location of pain and absence of a palpable nodule or "triggering" sensation, but it remains a plausible diagnosis.
Thumb Sprain: Although there's no reported trauma, a minor sprain could cause pain and limited mobility without significant swelling or bruising, especially if the injury was not severe enough to cause noticeable deformity or instability.
Osteoarthritis of the Thumb: Early stages of osteoarthritis, particularly affecting the carpometacarpal joint, could present with pain and decreased mobility without significant swelling or deformity. However, this would be less common in the absence of other symptoms like morning stiffness or crepitus.

Gout or Pseudogout: Although less common in the thumb, these conditions can cause acute, severe pain and should be considered, especially if there's a history of similar episodes or if the pain is unusually severe. The absence of swelling or redness does not rule out these conditions entirely.
Infection (e.g., Septic Arthritis or Osteomyelitis): Any infection in the thumb could present with pain and limited mobility. While the absence of fever, swelling, or redness makes this less likely, it's crucial to consider, especially if the patient has risk factors for infection.
Tumor: A bone or soft tissue tumor could cause localized pain and decreased mobility. Although rare, this diagnosis is critical not to miss due to its potential impact on prognosis and treatment.

Ganglion Cyst: While more commonly associated with the wrist, a ganglion cyst could theoretically occur near the thumb, causing pain and limited mobility if it compresses nearby structures.
Sarcoidosis: This systemic disease can cause musculoskeletal symptoms, including joint pain and stiffness, though it would be an uncommon cause of isolated thumb pain.
Reiter's Syndrome ( Reactive Arthritis): This form of arthritis can follow a bacterial infection and cause joint pain and swelling, but it's rare and usually involves multiple joints or other systemic symptoms.