37, mom & sister had breast cancer, found a lump

I'm 37 years old. My Mom had breast cancer 2 years ago and my older sister is currently undergoing treatment. I've always had regular periods with no breast pain or changes. I have quite small, not very dense breasts. 2 days ago I woke up to my period and both breasts were tender. The left one was also quite sore. I regularly check myself in my shower out of paranoia and have never felt anything suspicious. When I felt my breast I felt almost like a pea just below my nipple just below the skin. It hurt to touch it. I'm having a particularly heavy period and the lump seemed to grow throughout the day.

Went straight to my doctor and she got me in for a mammogram and ultrasound in 5 days. I've read so much about cyst vs cancerous lump, but because of my family history I keep going back to cancer. But for this lump to appear so suddenly on the day of my period, for it to be painful, close to the skin etc....I don't know! My doctor said she couldn't tell if it's a breast issue or a skin issue. She's a great doctor and obviously doesn't want to say anything until the results which is understandable. Has anyone had anything similar?