Hi
Welcome to the MedHelp forum!
Lump behind the ear that does not cause any other symptoms could be folliculitis (hair follicle infection), lipoma (localized collection of fat), a cyst, swollen lymph node or a neurofibroma (localized harmless swelling in of nerve and connective tissue). However swollen lymphnodes behind the ears could be due to chest infection, tonsillitis or pharyngitis, dandruff, or infection of ear. So check and see if there is dandruff or rash or if the baby has ear infection.
Watch the lump. Do not touch everyday but once in 10-15 days. If the lump increases in size, becomes painful, changes color, bleeds, changes shape or consistency or more similar lumps appear then consult a doctor.
Hope this helps. Please let me know if there is any thing else and do keep me posted. Take care!
My son has the same thing with no pain or anything else but he do have dry scalp should I try to find a treatment.