What Caused My Dog to Die Suddenly?

I had a 7 year old German Shepherd who was fit and healthy as told by the vet. I took him in for a check up around 2 weeks ago as he had injections for the kennels as we were going away.

Last Sunday he was all playful and happy, being his usual self! There were no signs of stress or unhappiness! Around 9 o'clock we brought him in the house ready for bed, he had his food/water (dogs biscuits and chicken), and went back out for a toilet then came back in! At around 12 o'clock (night) my mum heard him panting, but not badly just as if he was thirsty or really warm. So she let him out and he came back in and was fine. We woke up around 6 am to have seen 2 sick patches (one green water and other a big lump of food). We found our dog laying in his normal place/position, but obviously he had passed away. (Thinking passed away in his sleep.) So we just wanna know what people think it could have been. We are all heartbroken that our best friend has gone and it's so tough knowing that's hes not here. Any help/ideas would be lovely.