What Causes Cold And Fever With Lump Under Jaw Line?

My 3-year -old daughter has a cough, cold and slight fever. The following morning, I found a hard lump under her right jaw line. I brought her to a dentist but according to said dentist, it is not associated or connected with her teeth or whatever. So, I brought her to her doctor who gave her antibiotics. I brought her for a follow up check up but the lump was still there with the same size. Thus, lymp node theory was set aside and considered another theory, mumps..After two weeks, she was again brought to the doctor but the lump was still there with the same size and location..She still has her cough and cold. Her daily activities were not affected at all. She s very active and eats well. She s not bothered at all..Being the father, I am the one bothered. What could it be, doc?