What would be issue of my lymph nodes swelling by neck and behind knee?

Perhaps many: Enlarged lymph nodes usually is indicative of an infection. However, the lymph system can be affected by certain tumors such as lymphoma. The separate distances of the enlarged lymph nodes would concern me and i would recommend you be seen by your pcp. You may need to see a surgeon for biopsy and you may need further testing by bloood and imaging studies. Fever? Weight loss? Other symptoms?

Any other symptoms?: Many things can cause swelling of lymph nodes. Other symptoms you are having may help differentiate between them. Do you have symptoms like fevers, weight loss, night sweats? Infections, inflammatory conditions or some cancers like lymphoma can do this but most lymph node swellings are not cancer.