Level IA and IB painless lymph nodes.

6 months ago I had a painless hard insignificant(I felt) occipital lymph node.After sometime I had hard immovable very insignificant post auricular lymph node . After sometime I had mobile painless submental and submandibular lymphadenopathy.I consulted a MD medicine doctor and he advised me to do a USG.After analysing the report he said these cervical lymph nodes are benign and they will reduce in a couple of months but they didn't reduce.No other symptoms present.No oral suspicious leison no fever nothing.I started smoking at 21 years of age and quit in 24 years of age. What could it be doctor??plz reply I am really worried.? I went two days ago to that same doctor and he said these are nothing you are only thinking more...can it be idiopathic lymphnodes or may be a cause of more serious systemic illness???.