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It sounds like your dog has a condition we call HGE (hemorrhagic gastroenteritis). Basically it's where there is bleeding of the GI tract and the diarrhea ends up looking like jelly or red water. The cause is often unknown but likely either infectious (bacteria/ viral) or stress induced.
Diagnosis is made on rectal exam and then checking a PCV (packed cell volume or red blood cell percentage). In normal dog's it's 35-50%. In dogs with HGE that number is > 60%.
Treatment is by rehydrating as they lose a lot of water and electrolytes into the colon. We usually put these dogs on IV fluids and IV antibiotics for 12-24 hours. Most will have a full recovery with aggressive treatment. Without treatment, some will resolve on their own but it can be fatal in severe cases if not treated.
I would get this dog seen as soon as possible with your vet or an ER vet to get the diagnosis confirmed then treatment started. If he's not vomiting, then oral antibiotics and some SubQ fluids may be all that is needed.
I hope this helps, let me know if you have any other questions.