Wow
It is something i have oinly noticed once while petting her and havent been able to find it again, if i happen to notice it again it will be a trip to the vet.
Fatty lipomas are usually not a health threat, just cosmetic. They have been linked to low thyroid as well though, which is a health risk. You may want your Vet to check her thyroid levels. Also try to keep any extra weight off her too.