fatty lipoma

My guinea pig Peach (2 years old, female) recently developed a lump on her back/booty area maybe 2 weeks ago. I, of course, took her to the vet, though she seemed (and continues to seem) absolutely fine - eating, drinking, zooming, etc. The vet first attempted to drain the lump, in case...

So I suppose I should start from the beginning, shouldn't I? So a few months ago (maybe five or six) I noticed a small lump, about the size of a pea, on my cavy's side. I was a little panicked, because she's one of the first guinea pigs I've ever had and I had never experienced something like...