I've Recently Developed A Very Tender Very Large Lump Under

I've recently developed a very tender very large lump under my left nipple. It's definitely breast tissue, though I don't know if it's technically gynecomastia. I had the same issue twenty years ago, when I was taking a supplement with very high caffeine content. It resolved when I stopped taking the supplement. When I realized that I had again developed this problem, I stopped eating foods even moderately high in caffeine, such as green tea and chocolate. It's been a month now; the swelling is less sensitive, but about the same size. Do you have any suggestions? Thank you