J.B.
I also agree to call your PCP. She or he will do an exam and then will more likely than not refer you for a mammogram or other imaging even if it's most likely a cyst. I had a lump a few years ago that was a clogged milk duct. I had a mammogram and ultrasound to confirm, but it was terrifying.
If it's just a cyst, when this is over you may want to consider thermography. It's not embraced by conventional medicine but there are a lot of people in the "alternative" medical fields who advocate for this, especially with a family history of cancer. A series of scans now can set a baseline level of activity in your body and then you get re-scanned at regular intervals. The idea is that when comparing scans over time, a thermographer can pick up areas of possibly cancerous activity long before those cell changes build into something discernible like a lump. Unlike mammograms, thermography doesn't use radiation and doesn't touch your body, where a mammogram can actually damage breast tissue.
My mom has a lot of cysts in her breasts and has had many, many mammograms. She is concerned about the radiation and tissue damage of frequent mammograms so she is planning on doing thermography this year to cut down on the frequency of mammograms. Here is a link with more info if you're curious:
http://www.breastthermography.com/
Best wishes to you - I really hope it turns out to be nothing to worry about!