Reactive Arthritis or Post-Surgical Arthritis: This condition is a common reaction to surgical stress, especially after a major surgery like hip replacement. The body's immune response can lead to inflammation in multiple joints, causing swelling, heat, and arthralgia.

Infection (Prosthetic Joint Infection): Although less common, infection of the prosthetic joint is a significant concern post-surgery. Symptoms can include localized signs of infection such as redness, warmth, and swelling around the surgical site, as well as systemic signs like fever.
Crystal Arthropathy (Gout or Pseudogout): These conditions can be triggered by the stress of surgery, leading to sudden onset of joint pain and swelling. Gout typically affects a single joint initially, but pseudogout can affect multiple joints.
Rheumatoid Arthritis (RA) Flare or New Onset: While less common, the stress of surgery can trigger a flare in patients with pre-existing RA or even lead to the new onset of RA in susceptible individuals.

Septic Arthritis: This is a medical emergency where a joint becomes infected with bacteria. It requires immediate attention and treatment to prevent destruction of the joint and potential systemic complications.
Deep Vein Thrombosis (DVT) or Pulmonary Embolism (PE): These conditions can present with pain and swelling in the legs and are significant risks post-surgery, especially after hip replacement. They are life-threatening if not promptly diagnosed and treated.

Serum Sickness: A rare immune system reaction that can occur after exposure to certain medications or foreign proteins, leading to symptoms that include arthralgia, rash, and fever.
Hematoma or Bleeding Disorder: Although rare, a significant bleed into the joint or surrounding tissues can cause swelling and pain. Underlying bleeding disorders can increase this risk.
Lupus or Other Autoimmune Disorders: In rare cases, surgery can trigger the onset of autoimmune disorders like lupus, which can present with polyarthritis among other systemic symptoms.