I think it must have bothered her because it would often have a fresh scab on it, as though she was biting at it. I gave up figuring it would ever heal and had it removed today. I'll stick a few pics up and put links to others in case you're interested. As the title notes, it's likely a trichofolliculoma (lump -- tumor just under the skin). I asked for it back and sliced it open. Definitely not a cebaceous cyst nor an abscess.
The newest Ferrets, Rabbits, and Rodents -- Clinical Medicine and Surgery, says that the most common tumor in gps is the "trichofolliculoma, a benign basal cell epithelioma."
"Trichofolliculomas appear as solid or cystic masses, most commonly over the lumbosacral area. They are easily removed surgically."
Anyway, I chose not to have it biopsied as they are usually benign and I would be unlikely to do chemotherapy. Snowflake is about 6 years old now.
First noticed:

Lump two days ago, still the same size but I think she was biting at it more:
www.guinealynx.info/photos/lump-2.jpg
Side view of stitches:
www.guinealynx.info/photos/lump-3.jpg
Closeup of stitches:
www.guinealynx.info/photos/lump-4.jpg
Top of removed lump:

Bottom of removed lump:
http://www.guinealynx.info/photos/lump-6.jpg
Lump sliced in half and kind of doubled over (both sides of cut visible):

Second view of sliced lump:

I don't know if anyone is interested in these pics but thought I'd post them here.
Snowflake is eating well and seems like herself.
I dabbed some Chew Guard around the incision area in hopes that she will not bite at it. This is Snowflake's second surgery (she was spayed). She was a trooper through it all.