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.