Painful Lump in Armpit with Period

I noticed a lump under my right arm for the first time about 8 years ago, when I was 18. My doctor dismissed it entirely, so I wasn’t too concerned. I went several years without medical care until I finally just obtained a new primary, at age 26. I informed her that I have had a lump under my right arm that comes about 1.5 weeks before my menstrual cycle for years. It’s gone within the first day or two of my cycle but it has become increasingly painful in the past few months. If I sit upright with my arms to my side, you can see the lump stick out about half an inch. It’s squishy on the surface but hard when you press down. The doctor examined it and immediately sent me for an emergency ultrasound. The hospital doesn't typically take ultrasound appts on the day I was there but she expressed the urgency and they were able to see me. The ultrasound tech struggled to find anything but eventually did. She left and came back in with a Radiologist who said it looked like I could have an infection and asked if I had swelling anywhere else, such as on my neck. I explained that I didn't and explained the connection to my menstrual cycle. She explored further with the ultrasound and said my lymph nodes appeared normal. She then sent me immediately to have a mammogram just to verify that everything was ok.They noted that I have extremely dense breast tissue but otherwise it was normal. They said it looks like it's just accessory breast tissue and that the pain is due to hormone changes and nothing to be worried about. They did all my blood work including checking my thyroid which all came back normal. I just can't believe this is normal and am so nervous that something is being missed due to the dense breast tissue. I also worry that the part of my armpit wasn't captured fully in the mammogram. There is a history of breast cancer in my family. Am I over thinking this and if not, what could it be and what should my next steps be? They did say they could refer to a surgeon to have it removed only if the pain is bothering me too much, which I wouldn't do unless medically recommended. Any guidance is appreciated! I am worried sick!