I still have swollen lymphnodes in my neck after tonsillectomy 13 days ago? Been swollen 3 months

Need a post op visit. Ask your doctor to check this during your post surgical check up. You can review your history of taking antibiotics prior to the surgery and weather you had a culture done before surgery or during surgery. It may take a while to resolve since you have had the enlarged lymph node a long time. Aso tell the doctor if it is still tender or if that has resolved.
Viral. You may have a viral infection and your body has responded by causing lymph node swelling to fight the infection.