Unfortunately, a lot of vets aren't guinea-savvy (I drive an hour true to take my pigs to a Guinea-Pig Clinic when they need seen. Thankfully rare, but when my GP2 was ill, and my GP3 needed neutered, we did the trip)
Guinea-pigs can get diabetes like any other mammal, maybe thats why they were asking about drinking alot?
I'd start with the basics (when you hear hoofbeats it's more likely to be a horse than a zebra yes)
Skin is likely to be mites, lice, fungal or ringworm.
Wash her with a treatment shampoo (look at the Gorgeous Guineas website, email them for advice and look at their photographs). Mites can give a snapped off hair shaft.
You are best to treat both pigs and change all the bedding. (I steam my Pigs house and use Vikron S spray solution)
I use Xeno 450 ( though I know not everyone recommends it as some pigs react).I have to make sure mine don't groom each other , it's the boar who does this mainly. You can't treat and shampoo together, I'd leave a week or two between.
Ringworm isn't treated by Xeno 450 but mites and worms are (my hogs graze so I want to protect them).
My boar went through a soft pooh phase, I got some probiotics and Vit C to boost him, combined with cutting his veg and upping his hay/pellets.
We will try anything for these animals, won't we.
You could wait till you see the other vet , they might do a scrape and have a look for mites, but as you say, if you start blood tests and scans , you are looking at ££ and then what? If they find anything, do you want to go down the route of treatment?
(Cover your ears guinea-pigs but IMHO, their quality of life comes way ahead of their quantity. I;d much rather they have 3-4 happy spoiled years than 6 years of vet trips and medication/surgery. That sounds clumsy but hopefully YKWIM)
Would you be able to put a photo of guinea -piggie on MN and we can all hopefully shed some light for you?