Hi,
I can only give you my experience. I, too, was diagnosed with non-hodgkins lymphoma. I only had one bump and it was on my scalp just under my hairline... nowhere near a gland. It was never sore, I never had any kind of sympton other than a little fatigue. So, in my experience, I had very little symptons, but still had lymphoma, so it can happen.
Also, about the original bump and biopsy... I had to have 3 separate biopsies done before a diagnosis could be made because non-hodgkins lymphoma is very hard to diagnose with a small biopsy (or so I was told). If they were unable to get enough of a biopsy then the Patholigist may not have been able to analyze it well; HOWEVER, he/she should have said it was "inconclusive" rather than "non cancerous" if that was the case. A bone marrow biopsy that comes back negative does not mean she doesn't have lymphoma; it will only mean that it has not spread into the bone marrow. Usually this test is done to stage the cancer.
About waiting 3 months... lymphoma is a "slow" spreading cancer compared to many other cancers. She may not have lymphoma at all, but if she does, the results of all the other tests indicate it is in a very early stage and will probably remain there for a while. HOWEVER, and this is a big point, any time she experiences discomfort or symptoms she should IMMEDIATELY report it to the doctor. Finding a lump would fall into this category. She should definitely call the DR and discuss this find with him/her. It may only be an infection, but let the DR decide that.