Thank you for the additional information.
It is very possible that this is due to eating bad food or getting in to the compost pile. Most owners are unaware that compost piles can be very dangerous as they harbor potentially fatal bacteria. I would try to make sure the dogs do not have access to this area in the future. Bad and or moldy food from the trash can also cause dangerous garbage toxicity.
Please read over the following information so you can see what other symptoms you would want to watch for LINK
Bloody jelly like stool can also be cause by eating something toxic other then garbage such as rat poison
These are just a few of the possible causes.
The heat can cause problem with the blood clotting and heat stroke can happen very quickly so if you see signs that they are over heating bring them in. We had 4 heat strokes in our ER just this week and all 4 of them were just on nice easy walks with their owners.
What I would suggest if he is not vomiting is to offer a bland diet of boiled chicken and rice for the next 2-3 days then slowly go back to his regular food. I would keep a close eye on him. If the diarrhea gets worse, you see more blood, he has any vomiting, refuses to eat or you see any of the neurologic signs on the information I refereed you to regarding garbage toxicity I would have him seen right away.
If he continues to act and eat normally but the diarrhea continues then I would have your vet take a look at him on Monday and have the stool checked for bacteria and or parasites.
I hope this helps. Please reply back if you have additional questions or concerns and I will be happy to continue.