What Causes Swelling Of Glands On Neck?

my daughter is 8 years old and was diagnosed with a dust mite allergy and dust allergy.She has suffered with her glands coming up on her neck since having her tonsils and adenoids removed.I have consulted 2 different gps and she has had blood tests to check for other conditions these came back normal and I was told the pea size lumps were due to the body trying to fight off the allergy .Is this true ? of have i been fobbed off twice ? I am concerned as they have been there a while.They are always the same size never bigger just the size of a pea and they arnt hot to touch or red she says they can be tender if they are pressed otherwise she is unaware they are there .they are visable one day and then we wont see them sometimes they go quick sometimes they stay a few days .She has seen a ent specialist who ran the blood testing to diagnose her with this problem and this appointment came about as the gp refered her to him after us complaining that her nose was constantly blocked on one side .is this just something she has to live with nasonex helps but doesnt last long !!