Septic arthritis occurs when bacteria or another infectious agent is introduced into one (or more) joints, leading to painful inflammation. The signs of septic arthritis include heat, swelling, and pain in one or more joints. There will typically be decreased range of motion in the affected joint(s), as well as fever, lethargy, and lack of appetite.
https://vcahospitals.com/know-your-pet/septic-arthritis-in-dogs