Hi there. Well, you may have some mild bruising. The machines they use for mammography do squish and squeeze us and perhaps your technician wasn't as gentle as she should have been.
Give it a couple of days and it should feel better.
Is the skin sore and tender as well as inside?
I found I was allergic to the cleaning fluid they use to clean the metal sides of the machine.
You do not say when you had the mammogram. Personally I would make an appointment and see the doctor. There is a different way of checking for breast problems, I think it is an ultrasound, discuss this with your doctor. I think they do it every 3 years, not sure. I do not need to have any more done because of my age.
Have a look at the skin of the breast to see if there is any irritation, redness as well as the soreness. Make an appointment and see your doctor, it should not cause so much discomfort.