The jelly diarrhea and foul smell make me think intestinal distress. My puppy went through this at around 7 months. I think some bad treats set it off. She too had a very jelly like diarrhea, dark almost red that smelled horrendous. It came on quickly. She was given an anti-nausea and anti-diarrhea medicine by the emergency vet. The scary thing is she had become dehydrated. They also gave her fluids overnight.