Abnormal poops are usually caused by a digestive imbalance - often too much grass or veg. Piggies need a high fibre diet and their gut bacteria are geared for that, so if that balance gets knocked off (either through illness, medication, too much watery veg) , then the poops stop forming properly.
With a minor imbalance, the poops will still be formed to some extent but may be softer than normal. This can be resolved by taking veg off the menu, feeding more hay, giving a probiotic and allowing the gut time to rebalance. This usually happens within a couple of days. if it doesn’t happen and poops remain abnormal, then it’s possible something else is going on and vet care needs to be sought.
The problem with full liquid diarrhoea is the huge risk of dehydration which is why it becomes an emergency and require urgent vet care (along with the measures above) - subcutaneous fluids, pain killers, gut stimulants etc may be given.
However, if there are other symptoms then a vet needs to be seen rather than just relying on taking them off veg because the sooner any underlying cause is dealt with, the better.
Digestive Disorders: Not Eating - Diarrhea - Bloat - GI Stasis (No Gut Movement)