Breast lump?

So yes I am calling my dr for an appointment but does anyone know anything about lumps in the breast? Like if it’s sore is that a really bad sign? And deep too. When I say lump it’s like ping pong ball size but not round more football shaped if that makes sense. I thought I just had sore boobs during the week before at till I realized it’s not both boobs it’s just one and it has this lump, it’s to the side close to my armpit but deep in the breast tissue. Anyone have any info for me? Tia.