Dog has fatty lump on posterior?

I noticed this lump about ten days ago. I will sound like a moron saying it appeared out of nowhere, but I brush this dog regularly because of her shedding, and she had a bath 2 weeks ago and it simply was either not there, or too small to be noticed.

6 1/2 year old Brittany/Lab mix female. UTD on shots, worming, spayed. Dog is slightly overweight (5-6 lbs) in spite of daily exercise consisting of 1 mile walks and unlimited play on my 3 acres. Lump on her lower hip- trending more towards her backside. Lump seems to be contained just beneath the skin- not into the muscle tissue. Lump is approximately the size and shape of small egg- very fluid and flexible mass, almost seems to slither under my fingers when I grab it. The perimeters of it seem stable but are not an absolute line. I grabbed it hard a couple of times to feel the texture (which is very smooth and consistent- no hard spots or weird irregularites no marks on skin, no hair loss) - dog didn't even turn her head. It does not cause pain. The dog ignores it- does not chew or scratch or favor the leg. Dog has no weight loss or other physical issues at all.

As a shelter volunteer, I have access to their vets and counsulers. General consensis: This is not an abcess. This is not an infected wound. This is not a tick bite site. This does not present like a tumor because of its uniform shape, smoothness, and flexibility. Diagnosis: fatty cyst.

I intend to have the dog checked over after the holidays- shelter personel think she'll be fine until then. The reason I would like to wait is because I need to make the xtra holiday pay to cover a full exam. HOWEVER- if anyone feels like there is something I should be more immediately concerned about, if I described anything that triggers an alarm, please give me input.

Thank you- all serious answers appreciated. (Please don't waste your time with "Take it to the vet"- she has seen a vet and will soon see another.