Lactation adenoma but doesn’t make sense

One week after having my baby I started with mastitis. Since this is my third baby I'm very familiar with the feeling. As the result of constant massaging to express the milk, I found a lump in my armpit. I was confident the mastitis was originating from another part of my breast, where it was hot and red- not the armpit. Anyhow! I kept an eye on the lump and noticed it didn't go away. I scheduled with my GYN but had to wait to get an appointment which put me at 8 weeks since noticing the lump. She immediately said it was a lymph node and sent me for an ultrasound. The technician finished my test and stepped out to speak to the radiologist. When she entered back into the room, she said they "THINK" (she stressed the word) it's a lactating adenoma but I'm to come back in 6 months for a follow up. She said if anything changes in the meantime, I'm to come back in immediately. Now my questions- is it common practice that they would do a biopsy to determine for sure? My lump is hard, no pain, the size is 1.2cm x 1.3cm x 0.8cm.

It seems the lactation adenoma is commonly located on or within the breast- I'm struggling to find anyone who had had one under the armpit. Does anyone have an idea if this is more concerning than just waiting for a follow up in 6 months?

Thank you

Ps- no immediate family history of breast cancer but my cousin has had it- and many other family members with other forms of cancer.