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It is difficult to comment without examination. Lump behind the ear could be an abscess, folliculitis (hair follicle infection), lipoma (localized collection of fat), a cyst, an inflamed lymph node or a neurofibroma (localized harmless swelling in of nerve and connective tissue).
However swollen lymphnodes behind your ears could be due to chest infection, tonsillitis or pharyngitis, dandruff, infection of ear or even metastasis from a cancer. It could also be of tubercular pathology.
Since you have stuffy nose and sinusitis like symptoms, you must consult an ENT regarding this.
Hope this helps. Please let me know if there is any thing else and do keep me posted. Take care!
I think you should see your physician. You may require an ultrasound and/or biopsy to evaluate that lump further.