I would suppose a physical examination by the doctor for lumps in the neck, tonsils, ears, chest was normal at the time you took her to the doctor.
With season change at this time of the year, viral infections are common and kids will pick them up from school.
However, this lump if in front of the neck or rather to one side may be lymph
nodes that is the body's response to infection whereby cells multiply in these glands and reach the site to ward off the infection. This takes time and hence you have found these now. You most certainly should take her again if she is not on antibiotics and fever