Survivor since 2009
The first step would be to see your doctor. They will examine the breast to see if there are any palpable lumps. The next step would normally be a mamogram, followed by an ultrasound. Sometimes, in young women, the doctor will bypass the mamogram and go right to the ultrasound. A biopsy will only be ordered if there is some kind of suspicouse mass.
By all means see your doctor as soon as possible when there is any question.