Cellulitis: This is a common cause of soft tissue swelling and elevated CRP. Although the WBC is normal, CRP is a more sensitive marker for inflammation, and its elevation suggests an infectious or inflammatory process. Cellulitis can occur without an identifiable cause, such as a break in the skin.

Prepatellar Bursitis: Inflammation of the bursa in front of the kneecap (prepatellar bursa) can cause swelling over the tibia if it extends downward. It might not have a clear cause and can present with elevated CRP due to inflammation.
Tibial Stress Reaction or Stress Fracture: Overuse or repetitive stress on the tibia can lead to a stress reaction or fracture, which might cause soft tissue swelling. Early stages might not show up on X-rays, and elevated CRP could indicate the body's response to bone stress.
Insect Bite or Sting Reaction: Some reactions to insect bites or stings can cause significant soft tissue swelling and an inflammatory response, leading to elevated CRP.

Osteomyelitis: Although less common, osteomyelitis (bone infection) is a serious condition that requires prompt treatment. It can present with soft tissue swelling over the affected bone and elevated inflammatory markers like CRP, even with a normal WBC count.
Deep Vein Thrombosis (DVT): DVT can cause swelling in the leg and might not have a clear cause. While it typically presents with other symptoms like pain and warmth, it's crucial not to miss this diagnosis due to its potential for serious complications like pulmonary embolism.
Erythema Nodosum: This condition involves the formation of nodules under the skin, typically on the shins, and can be associated with various underlying conditions, including infections and autoimmune diseases. It might present with elevated CRP and could be considered in the differential for unexplained leg swelling.

Sarcoidosis: A systemic disease that can cause inflammation in various parts of the body, including the skin and bones. It might present with soft tissue swelling and elevated CRP, although it would be an uncommon cause.
Lymphoma: Certain types of lymphoma can cause soft tissue swelling due to lymph node enlargement or direct infiltration of the skin and soft tissues. This would be a rare but important consideration in the differential diagnosis.
Panniculitis: Inflammation of the subcutaneous fat, which can cause soft tissue swelling. There are various forms, some of which are associated with systemic diseases. It's a less common condition but should be considered in the differential for unexplained soft tissue swelling.