Hi, can you post the full text of the ultrasound report?
There doesn't seem a lot of cause for worry at this point. The mass looks the same to the ultrasound as the surrounding tissue, so it is probably composed of the same tissue. If there is a lot of fat in that area, then a lipoma (a not-cancer fatty mass) would be difficult for the ultrasound to see. (That is not the same as a lymphoma. Totally different.)
Since the location is in the armpit, one naturally thinks of metastasis from breast cancer. HOWEVER, a metastasis *would* be seen by the ultrasound. So it's not a metastasis. But still, I'd ask for a needle biopsy to see what the bump is composed of.
Also, a lymphoma would almost certainly be seen on ultrasound.
If she is rubbing it a lot, that irritation can cause some swelling.