15 year old with large painless lymph node enlargement

My 15 year old daughter has 2 enlarged cervical nodes one of which is LARGE (bout the size of the first digit of the thumb). Family MD did some labs that included EBV and CMV both of which were negative. CBC only showed WBC low, NE% low, LY% high, MO% high and AST high. Abdominal ultrasound showed mild splenomegaly.The only abd pain she has complained of was a couple of mornings she said she feels a tightness in her upper left quad with a little pain but it subsides as the morning goes on. She has absolutely no other symptoms of illness (No fever, no sore throat, no URI, no ear pain, and has no tonsils.) She has been a little fatigued and some diarrhea has developed over last few days. Not exactly sure how long they have been enlarged but we found them over a week ago.
Family MD thinks it is related to Mono she had 2-3 years ago. Not so sure I agree with that but he is the MD.  He put her on a round of steroids and they have went down very little. Usually when she gets up in the morning they are alomst back to the huge size but as she is up and about they go down a little. She has had a couple of bouts of waking up breaking out in sweats, but only a few times.
She is a very active, athletic, rarely sick child but I am concerned this might be more that just a 2-3 year lingering Mono.  Back when she did have the mono her glands were no where near the size they are now and they were only that way for a few days and she had all the full blown signs and symptoms of Mono.
I was looking for some insight as to what this could be or other measures / avenues we my need to explore.  
We follow-up with family MD this Friday and not sure what what he might suggest.
Is it possible to have some sorta lymphoma and lab work be pretty much normal. Should we be concerned that this is possible.
We have a strong family history of different kinds of cancers also.

Any comments on this will be greatly appreciated.
Thank you