Girls, my breast is hurting on the left side, lower down near the righ…

Girls, my breast is hurting on the left side, lower down near the right side of the armpit, there's a lump I didn't notice when I touched it, and it hurts when I walk too. I have an appointment with a doctor on Monday, but... All sorts of awful thoughts come to mind, what could this be? ? Yesterday evening I noticed that it started to hurt. Maybe tight underwear pressed against it, or something more serious? Milk is flowing, the baby eats from this breast.