Why did my guinea pig die? Very sad.

Question
On Sunday October 3rd Wilber seemed very happy and was his normal self. On Monday he seemed just fine and Tuesday he was a little quieter but didn't think much of it.  By Wed. night when we got home from gymnastics he was breathing shallow.  He let me just take him out of his cage with out being feisty like her normally was.  He just lay there in my arms kind of limp.  He took a small nibble of romaine lettuce but wouldn't swallow it.  He let me pull it out of his mouth even.  By this time the vet was not open and I knew something was very wrong.  I went on line to check symptoms of a dying guinea pig and he seemed to have some.  I wrapped him in a towel and held him very close to my neck and chest and we snuggled.  With in a half hour he was gone.  Back in June he was treated for manage mites with ivomectin not sure of the spelling.  Could that have stressed him out enough to do this.  Otherwise he was very happy and healthy.  So sad and confused.  Please help.  I am just sick over this.

Answer
I'm sorry to hear of your loss.
Ivormectin would not of stressed him out. It is a commonly used drug to treat rabbits and rodents for mites, and he would of had side effects much sooner. The only circumstances that would of caused his death due to the mites is if they were that severe that he was in a terrible scabbed state and this would of cause stress and possible fitting and could of lead to his death

You donot say how old he was. Any guinea pig 4+ is getting on a bit but they can live 8 years depending on the individual

Other than age it could have been due to organ failure if for instance something was not working properly
He may have had an internal cancer
It may be something hes eaten accidently
Unfortunately you cannot tell unless you have him taken to the vet within 2 hours of death for a post mortem, which would be more traumatic for yourself.