Hi Thank you for your query It seems most likely due to her monthly vaccinations especially if it is at the same site. Other possibilities may be a lipoma (collection of fat) or a neurofibroma. All these are benign so you need not worry. Consult a pediatric surgeon. Mostly it can be diagnosed clinically. So we will know whether we can wait or should go ahead and get it removed right away. In doubtful cases a test called FNAC can also be done in which just a small needle is introduced to know what the diagnosis is and further management depends on that. Hope that helps God bless you