Hello dear!First of all let me tell you that neck masses in children are benign in about 90% of cases.In this age group most important cause is inflammation followed by a very low percentage of neoplastic neck masses.Since your child is having lump on back of her neck so most common cause can be muscle or tendon injury
as neck stiffness
is commonly associated with this type of lump and secondly there is severe pain
on touching.Secondly it can be enlarged lymph
nodes and third possibility in this age group can be tonsillitis
, but it causes difficulty in swallowing also.Inflammatory lumps will regress once primary infection is treated wihin 4-6weeks but if it is persistent with or without symptoms then consult an ENT surgeon for complete examination.Stay cool and observe her for any other signs and symptoms.