What Causes Fever And Lump Behind Ear While Recovering From Stomach Infection?

My 14 month old is recovering from some virus that she got 7 days ago that consisted of vomiting, then high fever for 4 days, red throat and some white mouth sores. Her fever went away 2 days ago but her appetite has not returned (eating and drinking half of what she normally would). She has also been much crankier than usual and having an especially hard time going to sleep. Yesterday I noticed a lump behind her left ear (feels like a swollen lymph node) and today she has a low grade fever back - 99.5. Could she have a secondary infection? What do you recommend?