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If this was her hip...in some way, it would be painful and she would be limping.
If you look under the fur..is the skin red or irritated? I guess I am trying to figure out if the lump is within the skin itself and swollen like from a bite/insect sting...OR if it is deeper and under the skin and not involving the skin.
Well, if she is not painful, acting sick, or bothered by the swelling..I would leave it alone and watch. IF she starts to show distress or discomfort...you can look for an emergency vet...but for now I would just watch and take her in to the vet in the morning.
There are fat folds, lipomas (fat tumors)...there is not a lot that will do what you describe. It is not a common place for a herniation...at truly am at a loss for you. If it was swelling from an injury ...it would produce some pain.