Since you mentioned she has been ill with a fever earlier this week, I'd guess it is a lymph node... Perhaps a bit swollen from fighting off the recent infection. Probably good to have it checked, just to be safe.
Here are some links:
Swollen Lymph Node
Typically, it is caused by an infection that your lymph node is destroying. A slight enlargement and softness means the swollen lymph node is succeeding in the tough battle.
Here are some diagrams of the location of lymph nodes in the neck, scroll down to the bottom of the page: