Is it hard or soft? Reddened? Hot? On your upper chest or back? Tender to touch? Moveable or fixed? It could be many things---an enlarged lymph node if you had an infection before the onset (expected, not dangerous, as the lymph node "filters" and tries to capture the infection)...it could be a lipoma a benign, harmless fatty tumor...or it could be more serious, a lymph node enlarged due to blood cancer or a tumor. For these reasons, it's important that you see your doctor...to diagnose and treat if it is something serious and to put your mind at ease if it is something minor. Unless it is obvious on exam that it is a lipoma or other minor benign growth, your doctor will want to collect more information from you. Be prepared to answer the questions I listed above for your doc. He/She will also want to know if you've had any more generalized symptoms like susceptibility to infections (any cough, sore throat?), easy bruising or bleeding from your gums, night sweats or fevers. Wishing you a speedy resolution. Take care.
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A lump at the back could be a lipoma (localized collection of fat), a swollen lymph node, or a a neurofibroma (localized harmless swelling in a nerve). Folliculitis (hair follicle infection) can sometimes persist. It is difficult to comment without examination. Please consult your PCP even though all these are harmless swellings.
Hope this helps. Please let me know if there is any thing else and do keep me posted. Take care!