What Causes Infected Blister With Discharge On Breast?

I'm a 45 y.o lady with 8 kids, medically free with no family history of cancers.
I have a big discharging pimple on the right lower quarter of my right breast. It has been growing on size for the last few months. What could it be ? Knowing that i've had a mammogram & Ultrasound a year ago and they were totally negative