Bright red blood and mucus in soft stools/diarrhea

My cat (2 years old male, neuteured) has been having issues with his digestion for about 2 months now. He's been having soft stools and diarrhea since we invited some friends over and we think he's been very stressed. It may be the cause of this whole ordeal, we have no idea. We have consulted the vet about it.

They prescribed Fenbendazole for parasites and Hill's GI food. He took the dewormer a couple weeks ago and he's been on Hill's for over 10 days. I removed the water fountain, fearing it may be bacterias from the fountain, I also gave him Purina's Fortiflora probiotics which worked well in the past but didn't really help this time. Vet is considering switching his food once again to hydrolyzed food but I'm starting to think it may not be caused by allergies or food.

We always check him pooping when possible. He had a really great, formed poop yesterday and we were so happy about it. But he then had an episode of stress because we were working in our backyard and he was meowing and crying a lot. He's clingy and follow us everywhere, he's a very sensible cat. He's not lethargic, he's acting normal, is playing, is eating and drinking water.

Now I wanted to try a probiotic with S.Boulardii after reading a lot of other stories online. Can it help? We also put the bloody poop sample in a bag in the fridge so we can bring it to the vet as soon as possible (our vet is closed today but we're currently on a call with the emergency vet). The fact that he had a healthy poop yesterday and now blood and soft/runny stools is beyond me. Could it be stress? Could it be IBD? Could it be allergies?