Several things. The location & feel of the swelling can help define the cause. Many lymph nodes are located in the neck and feel like pea to peanut sized nodules unless they are swollen. The young infant may develop a knot in the main neck muscle on either side which restricts neck movement & makes it feel fuller on that side. All newborns have short necks & can just seem swollen (necks grow an inch or > by 6m).
Lymph Nodes. The most common cause is swollen lymph nodes which are always present in the neck, but appear larger if your child has an upper respiratory infection such as strep throat or an ear infection. If the neck appears swollen your child should be examined to determine the best cause and treatment.