With your family history and even without it any lump needs to be checked out. Could be a cyst, probably is, but it could be something else.
Only way to know is to have it checked and insist on a biopsy and not have a doctor say something like we'll keep a watch on it.
Keep up with your mammograms too.
My last Mammogram was in 2005, had one August 25th, 2009 I have breast cancer. Learned a hard lesson never had the slighest clue and it was the last thing I expected to hear.
There is no cancer in my entire family so you never know.
Keep us posted and a big welcome to HW. You have come to the right place. I'm sure others with a lot more experience than I will be along to advise you.