Large mass- hello I'm new and WORRIED!

Thanks for letting me join, and sorry, long post. I'm here like you all, I'm worried and need somewhere to talk about it. A little about me, I just turned 40, have no family history on my mother's side ( Ive never met her or know anything) my paternal grandmother had very early stage breast cancer late in life. They did a lumpectomy and radiation, but she had so many other issues going on, the small lump was the least of their worries.

I've always been super healthy, never go to the doctor for anything. I have one son who is almost 14 and I breast fed him for 18 months. Two years ago, my GP found an area of "thickening" in my right breast and she was somewhat concerned and sent me for a mammogram and ultrasound. There was a small area they found mildly suspicious so they ordered a biopsy. Everything came back benign and they told me not to worry. Fast forward 2 years, I'm now 40, in good health, other than occasional ovarian cysts, and I'm very active. Last week, I started my period which was normal. During my period, I felt that same thickening in my right breast. I didn't think much of it since I was on my period and my breasts always feel heavy/swollen during that time. Two days after my period ended though, I was in the shower and felt a very large hard area on the top outer side of my right breast. It's like the size of an egg. I panicked and got out of the shower and had my husband feel it. He felt it too. The next day I tried to get in with my doctor but found out that she suddenly retired and there are no other doctors available to see me so they suggested I go to urgent care because they wouldn't let me just schedule a mammogram. So I went to urgent care yesterday. The Doctor was a jerk, had absolutely no bedside manner. He came in gruffly, put his hand on my lump and said yep, feels like a golf ball and left the room. The MA came in like 45 minutes later and said that I have an appointment on March 11 for mammo/ultrasound. That seems like such a long time to wait! Of course I'm freaking out.... on a side note, since I found this lump 3 days ago, my armpit / shoulder has been uncomfortable. Do these things just pop up out of nowhere? I would have noticed an egg sized lump last week if it had been there. I'm super scared and can't stop thinking about the absolute worst.