hi thanks for posting in HCM I have understand your concern fever , pain in the back and knee nothing to do with appendix appendicular pain occurs over right lower abdomen along with vomiting and severe tenderness it may be a just viral fever and joint pains expected in viral fevers septic arthritis also one possibility final differential diagnosis should be TB joints no need to worry I can suggest you to get done CBC x-ray knee and troraco lumbar spine meanwhile take tab.hifenac -p twice daily for 3 days and depending upon reports we can decide further action of plan I feel it is a viral fever hope this helps you thanks