Non tender lump bottom of breast, below nipple? grape size longer than across, recent screening mammogram 2 months ago was clear, think it was there then but slightly bigger now?

Since this seems to have been present for a while and therefore has persisted through at least one or two cycles, further evaluation is needed to make sure this is nothing serious. A normal mammogram is reassuring but they do not pick up everything. An exam and a breast ultrasound would be your next steps. A breast biopsy would be indicated if there is continuing uncertainty.