Be wary of their vet! I got a hamster from Petco a year or so ago and he became ill pretty quickly. I called the store and they told me I could return him and they would treat him. I asked about their vet. Turns out they don't have their own on staff vet, they have a local vet come in ONCE A WEEK. They told me the name of this vet and it was a vet I had already called myself and they told me they didn't treat exotics. So not sure why Petco is using them to treat exotics (well, I guess I do know... cost...). The hamster ended up dying while they had him (he had bad wet tail, so I knew his chances weren't good).
It doesn't say much for them that they gave you your pig back with this very noticeable lump - either they are so incompetent they didn't notice, or they are hoping you won't notice. So if you see their vet, ask lots of questions: about the vets training and experience with guinea pigs, if they have treated this condition before, if they treat the pig there who handles the aftercare (like is there a store employee who is in charge of that or do they just shove them in a back room).
Good luck!