Hi Michelle, welcome! You have come to the right area, we'll help as best we can...
Myth #1: Breast cancer is painless.
Fact: Many of us had pains or painful lumps that turned out to be cancer...
I say that not to scare you, but to encourage you to get it checked out to the fullest. There are many many benign breast conditions that can cause pain, including changes associated with the menstrual cycle. You are extremely young for breast cancer, although not totally out of harm's way, the odds are overwhelmingly in your favor for this to be nothing to worry about
Your mom's family has a history of BC....were they young, like unde 40, when diagnosed? If many of them were very young, there could be a risk of a genetic mutation that would place you at potential higher risk...but if they were older, it may be nothing more than random chance.
Please insist that your doctor do an ultrasound. A mammogram is not likely to show anything in a woman of your age. They may wish to do an MRI or something too, don't be alarmed, but be happy they are taking your concern seriously.
You may also consider cutting out caffeine before the doc visit to see if that changes anything.