First, it's much too soon to worry.  There are so many different types of breast lumps, and most of the time even lumps that need to be biopsied turn out to be benign.  I'd recommend calling the doctor and setting up an appointment.  Depending on what he or she finds on the physical exam, the next step might be an ultrasound or a mammogram.  Sometimes, if the doctor is pretty sure it's a cyst, it's safe to watch the lump for a menstrual period of two to see if it goes away on its own.  If there is any doubt whatsoever about whether the lump might be cancer, a biopsy will be needed to rule out cancer.  But that's several steps ahead.  You've done the right thing by beginning to gather information.  Tomorrow call the doctor to set up that appointment.