Did she bang into anything or get hit over the weekend? I know my V once got rammed in the shoulder by another dog while they were playing and she woke up with a huge lump on her shoulder blade the next day. It was just swelling though and went down after a few days. I'd use a hot or cold compress on the area a few times per day for a couple of days and if the lump isn't shrinking in size, by let's say Wednesday, see the vet.
