lump that does not appear on mammograms or ultrasounds

Hello; My mother is 52 years old. SHe has had a lump under her armpit for 4 months. She had a mamogram (mammogram) and ultrasound 6 months ago in the same side for 2 cysts of 3 milimiters each one. She had a stereotactic biopsy 6 months ago in this area nd it was bening. Now she has had a mamogram (mammogram) and ultrasound and they were normal. Why this lump does not appear in these test.?
Do you think that she is going to have another biopsy ?
The lump is painless, it just hurt when the doctor touched it. The doctor said that maybe it could be that the breast is inflamated, Do you think that this could be Immflamated breast cancer ? The appearence of the breast according to the doctor is normal. For what I have read this coudl be an enlargement of the lymph nodes. WHy is this enlargement ? Do you think is cancer? What would you think is it ? My maother had cervical cancer 2 months ago, stage 1A.  My mother's grandmother had breast cancer when she was 75 years old. Please help  me, because I am really worried.

[CCF-RN,MSN-JS]

Dear Worried Son,  This lump could be caused by a number of things an inflammation of a lymph node doesn't mean, inflammatory breast cancer,  it means something may be causing it to become inflammed (swollen, painful), such as an infection.  Lymph nodes are small bean-shaped structures that store infection fighting cells in our body.  They are part of the lymphatic system that runs throughout our bodies.

An ultrasound is used to distinguish between a fluid filled or solid lump.  A fluid filled cyst is better distinguished on an ultrasound than a solid mass.  With a lump under the arm it may not be within the range of where the mammogram can pick it up.  

The next step of evaluation of this lump may be another biopsy.