I am really sorry for your loss.

It's always sad to lose a pet, especially suddenly and without warning and with no clear explanation as to why.
I have always read that the 'average' lifespan of a guinea pig is 5 to 7 years. But it's important to realize this is an average- to put it in human terms, it's possible to live to be 100 or more, but it's also possible to die of a heart attack at 45. When we average all those people out, we get an average lifespan in the mid-seventies, but that doesn't mean we are 'guaranteed' that span. Same with pigs, they don't all reach the 5 to 7 year point.
Our of my own pigs, one died of sepsis before the age of 2 (very sad!) One lived to be slightly over 6 before dying of what I now think was probably lymphoma, though never had a formal diagnosis. One is currently closing in on 7, the other is doing well at 2 years of age. So we've had a discrepancy in life expectancy, from some living into old age and one passing away quite young due to illness.
Small animals tend to hide symptoms and get sick very quickly, so sometimes by the time we see signs, the animal is already very ill. And, just like humans, some health issues like heart attack, stroke, or organ failure can crop up very suddenly with very little in the way of prodromal symptoms.
At any rate, I'm really sorry for your loss. ((HUGS)) to you!