My 11yr of dog just passed... why? did I kill him?

[cryinglikeababy]

Hi All, I was just wondering if you could give my some advise, help?
This time last week, I had a what I thought was a healthy  happy dog. Everything was fine throughout the day and evening, then at night he took a turn for the worse.. he went out for his usual evening wee and poo before bed, which he did and his back legs suddenly started to give way, he started breathing fast, and was sick. He carried on moving and came upstairs to bed. I heard him struggling to get comfy and he eventually went downstairs (unusal as he slept with us) he got some water but was sick and his legs went again, his paws was cold, he clamped his mouth shut and his legs kept dropping him within minutes he had either a fit or seizer and dead, his poor tonge had fallen out of his mouth and had gone very pale and he lost his life, I can't help but blame myself, did I miss signs? did I kill him as that day I had mixed a bit of jays fluid and used it on my washing line. I can't help but think I killed my dog and it's tearing me apart. I took him vets to be cremited (get his ashes back soon) but with out paying thousands I'll never know why or If I killed him but I will always have the guilt.
Has anyones pet died like this?