Have had swollen nodes on my neck for 3+ months. doctor said not to worry. no growth, no pain, no hardness, but they won't go away. possible causes?

How big?: A 28 year old generally should not have palpable nodes in the neck. It is good that they have not been growing. It is quite possible they are "reactive" , which means they became swollen in response to a viral or bacterial infection and never totally went away. In any case i would recommend a full head and neck exam by an ent. He may agree, no big deal, or may recommend a needle biopsy of node.

Needs further eval.: Depending on how big or what they feel like, they should probably be checked further. Chronic infection (like acne) or inflammation could give you chronically enlarged nodes, but they are often tender. You should be checked out a little further. This would include a good exam with attention to your other lymph node areas and abdominal organs, blood tests, and possibly a biopsy. Good luck.