Had 2 guinea pigs, sisters, 5 years old.
One was tiny and lost some fur lately (diagnosed with ovary cysts) but otherwise happy. The other was sturdy, looked healthy and ate for two!
All of a sudden noticed the sturdy one cannot move this morning and breathing heavily and felt kind of lumpy all over. Called vet & got appointment for 2 hours later. Tried giving water with syringe - she took none. 10 mins later, she was gone 
I am trying to look back - yesterday I thought the smell of the cage was a bit heavy (pee stink) but as I just cleaned it a couple of days back I thought that's a bit strange, but ok I will clean as soon as I can.
Other than that she was always the happy one, eating, moving about - although I cannot identify if she was well yesterday- the days blurred into one.
Anything I could have done? Anything I missed? Is that a normal passing after 5 years? No change in diet or circumstances... just hard not to blame myself that perhaps I didn't spot something on time.