Thank you everyone.
We took her yesterday to a very good little veterinary surgery. Thankfully, I've never had to take an animal to the vets before, so I didn't know what to expect. The lady who looked at Cinnamon seemed to know her stuff -- she had a good feel (plenty of wriggling and squeaking from Miss Cinny Pig) and said the lumps were attatched to the skin underneath. She said there was no easy way of telling what they were -- if they could get Cinnamon to hold still, they could use a syringe to get some cells to look at under the microscope, but that wouldn't tell us for sure whether it would grow. So we've been advised to take a photo of the lumps (have done so this morning), so that we can check her ourselves to see if it changes.
The vet told us it was unlikely that it could cause much trouble internally if it grew, but it could get uncomfortable on the outside -- rubbing on the ground and things, or getting sore. If we notice anything, we'll take her back. She said they weren't eager to go for the 'chop-it-all-out-and-burn-it' method, because she knew that guinea pigs and anaesthetic aren't very good friends at the best of times.
Cinnamon's back with Pepper now: nice and chirpy, happy as can be. And, fingers crossed, we won't be needing the big cardboard box with the newspaper and hay in the bottom again!
