Did your vet aspirate any of the lumps to determine what they were? There are a variety of non-cancerous types of lumps that guinea pigs can and will develop, including things like cysts.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry