Lyme Arthritis: Given the patient's lab results showing active Lyme disease, this is a strong candidate. Lyme arthritis often presents with joint swelling and erythema, particularly in the knee, and can occur in the absence of significant pain. The presence of a small joint effusion or synovial thickening on the X-ray supports this diagnosis.

Prosthetic Joint Infection: Although the X-ray shows no evidence of loosening, infection is a concern with any prosthetic joint, especially with signs of inflammation like erythema and swelling. The patient's slightly elevated sed rate could indicate an infectious or inflammatory process.
Crystal Arthropathy (Gout or Pseudogout): These conditions can cause acute inflammation of the joint, leading to erythema, swelling, and difficulty bending. However, the absence of significant pain and the normal CBC make this less likely.
Rheumatoid Arthritis: This autoimmune disorder can cause joint inflammation, but it typically involves multiple joints and is associated with more systemic symptoms. The patient's normal CBC and the lack of other joint involvement make this less likely.

Septic Arthritis: This is a medical emergency that requires prompt diagnosis and treatment. Although the patient's CBC is normal, and there's no clear indication of infection, septic arthritis can present subtly, especially in patients with prosthetic joints. The consequences of missing this diagnosis are severe, making it a "do not miss" condition.
Deep Vein Thrombosis (DVT): Given the patient's recent history of difficulty bending and the presence of vascular calcifications, DVT should be considered, especially if there are risk factors for thrombosis. DVT can cause swelling and erythema of the affected limb.

Hemophilic Arthropathy: If the patient has a history of hemophilia, this could be a consideration, especially with joint swelling and difficulty bending. However, there's no indication of hemophilia in the provided history.
PVNS (Pigmented Villonodular Synovitis): This rare condition causes joint swelling and can lead to erythema due to chronic inflammation. It's more commonly seen in younger patients and might not fully explain the acute presentation.
Neoplastic Processes: Rarely, joint swelling and erythema can be signs of a neoplastic process, such as a synovial sarcoma. These conditions are uncommon and would typically present with more systemic symptoms or a longer history of joint issues.