I just had an ultrasound of a lump and I don't really understand the findings?

[NatAncalade]

I started having pain in my right armpit in the extra breast tissue area. When I noticed the soreness I felt around and realized I could feel a sizable lump. There was no redness or itching and no fever so I didn't do anything about it for a couple days. It started to really hurt though so I went to an urgicare who sent me to the emergency room to have an ultrasound. I had blood work done which came out normal and even though I feel like I have a swollen lymph node in my neck they said it wasn't. These are the findings and they were not very helpful in helping me understand what the results of the ultrasound were.
Findings:
There is an irregular hypoechoic not parallel mass with posterior acoustic shadowing and no definitive internal vascularity measuring 0.9 x 0.9 x 1.2 cm
Impression:
1.2 cm mass in the right axilla corresponding to the palpable area of concern. This may represent an area of fat necrosis given axillary pain.
Recommendation:
The option was offered to the patient to undergo a diagnostic mammogram and possible ultrasound versus a short interval follow-up in 1. Month. The patient states that she prefers to follow up in one month. If this area is an unchanged or enlarging at the follow up appointment, ultrasound guided needle core biopsy is recommended.
I am just confused because they kept asking if there had been any trauma to the area and I can not remember anything happening that would cause this. The only thing that has happened on that side of my body was a huge bruise on my arm from an I.v. At a hospital visit that wasn't  catheterized properly and bled quite a bit and gave me a massive bruise, but this was about a week before the soreness started. I guess I am just really worried and want some assurance. Sorry for such a long post but thank thank thank you in advance!

[CHIN_C]

It is difficult to evaluate your case without images. However, according to WebMD, traumatic fat necrosis happens when there is an injury to the breast, though you may not remember an injury happening. It causes fat to form in lumps that are generally round, firm, hard, and painless.

Source: https://www.webmd.com/breast-cancer/qa/how-can-traumatic-fat-necrosis-cause-benign-breast-lumps