Y'know when you look in your guinea-pigs cage , what do you really NOT want to see there.
High on the list has to be blood stained pee patches.
This morning (our guineas had puppy pads and fleeces last night with a pile of hay in the sleeping area) the puppy pads had 6-7 fresh blood stained patches 
GP4 hadn;t shown any symptoms, no squealy hunchy peeing, no drips of blood, no light bleed. This was out of the blue full on.
We'd had to give her a bum dip on Saturday as she was a bit manky, but no blood in the wash.
So: whooshed her off to the Guinea Vet (with GP6 as company) and now she has antibiotics (suspect cystitis) but possible bladder stone.
(DH suggested she might've passed it already)
She did a couple of bloody pees in the Pighouse, but the pee she passed in the Vets was clear.
She's happy in herself, eating and bossing Bert round .
Only thing I can think of that might've set her off is I changed their food (I didn't think of this till I was driving home). Still Burgess Excell but its a new flavour (they've been scoffing it too)
I'll search out the original one and change back.
Poor girl 