Hi
It could be either runny diarrhea or if it is jelly-like or rubbery, it could be part of the mucus membrane at the exit of the caecum (the last and biggest of the fermentation chambers of the lower gut). It is there to catch gut microbiome from passing out of the gut. Very, very occasionally, some mucus slip through and once or twice we have seen this happen with a larger piece in guinea pigs with persistent digestive issues.
However, research into guinea pig digestion and nutrient/medication absorption and processing is only very recent and still very much ongoing. This is reflecting very recent research. Why this happens and what could be causing it are questions we can't answer but at least we know where those freak and usually one-off jelly like deposits come from.
This guide here explains the digestive tract in guinea pigs and how it works:
1 How the Digestive System Works
2 Poo Eating (Coprophagy)
3 Health Monitoring: Weighing vs. Poos Watching
- What does weighing do?
- What does the poop output tell you?
4 Minor Poop Issues
- How to deal with a minor tummy upset
- Funny poops: What do they mean?
- Caked on poops
5 Serious Diarrhoea
6 Impaction
7...
Please try to get as much feed into him as possible.