Hi and welcome
I am very sorry about the extremely long wait. Has the lump seen and been diagnosed by a vet or is that your own assessment?
Guinea Lynx :: Lumps
The good news is that where it is situated, it is not too close to any vital organs and should hopefully make for an easier and quicker operation. But it is obviously causing your piggy quite some discomfort as the area has been obviously barbered. Lumps in guinea pigs don't get smaller or disappear again. Some, like abscesses or sebaceous cysts can burst. Some can also put on a sudden massive growth spurt, which makes for an emergency op. All you can do is to keep an eye on it and push for the earliest possible date.
Here is our emergency advice as well as our post-op care link. You may want to read and bookmark them so you know where you can find the necessary information in a hurry and be prepared for stepping in with support care in case that becomes necessary.
Emergency, Crisis and Bridging Care until a Vet Appointment
Tips For Post-operative Care
My two oldest piggies needing an operation were both lump operations; Tesni needed a burst and infected but harmless sebaceous cyst removing that was located just next to her genitalia age 6 and Hafren's large cyst on her back (which we'd kept under observation for well over a year in view of her age) promptly picked up speed and burst during my vet's summer holiday absence. Hafren made it through her op a month before her 7th birthday and made a full recovery, the same as Tesni. Concerns of a possible tumour at the bottom of it by the local general vet I saw as an emergency did thankfully not materialise.
Tesni - Hafren on the day after their lumpectomy
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I hope that this helps you?