Oh I'm so sorry to hear that.
What medications was he given? Perhaps his immune system was weak and he succumbed to an infection that was deeper than the lump.
Vets for some reason usually don't have the expertise needed to treat small animals (or really anything outside of a dog, cat, or cow)...
Lumpy guinea pigs are very common. Probably a benign abscess from a bite. My guinea pig's lump healed after a couple of weeks but I would still recommend going to the vet so they can aspirate it and give you anything you might need to treat it.