Possibility of inflammation of pharynx and larynx should be taken into account in this case.Most likely cause could be reaction/trauma caused by trail mix.This inflammation can lead to enlargement of lymph nodes leading to stiff neck there by pain in neck.As throat is constantly moving(swallowing,respiration)inflammation takes longer time to settle than other parts.Start on warm salt water gargles,avoid smoking/alcohol(if any),drink plenty of fluids,take acetaminophen for pain.Antibiotics and steroids might be needed after consulting with your doctor if no relief.