Hi, The sudden onset of knee and elbow pain with tender and swollen joints could be indicative of several underlying conditions. Some possible causes include: * Arthritis: Joint pain and swelling are common symptoms of arthritis, which is an inflammatory condition that affects the joints. Juvenile idiopathic arthritis (JIA) is a type of arthritis that affects children and can cause joint pain and swelling. * Infection: Joint pain and swelling can also be a symptom of an infection, such as Lyme disease or a viral infection. The canker sore on your daughter s tonsil could be a sign of a viral infection. * Injury: If your daughter recently sustained an injury or trauma to her knee or elbow, this could be the cause of her pain and swelling. However, it is unlikely that an injury from last year is causing sudden onset pain now. * Autoimmune disorder: Certain autoimmune disorders, such as lupus or rheumatoid arthritis, can cause joint pain and swelling. It is important to seek medical attention for your daughter s symptoms, especially if she is experiencing a fever. A doctor can perform a physical examination, run diagnostic tests, and provide a proper diagnosis and treatment plan. While sepsis is a serious condition, it is unlikely to develop from scratch, especially if there are no signs of infection. Rheumatic fever is also a rare condition, but it is important to rule out potential causes of joint pain and swelling. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Priyanka, General & Family Physician