Hi
Changing hay can upset the gut microbiome for a short time until it has grown enough newly needed bacteria to cope with the demand; that may take a few days. It can also happen if your piggies don't like the new hay and eat less but you would see smaller poos and a short dip in weight as well.
Please always keep in mind that it takes about a day on average for any food to get from one end to the other, and then you need to make allowance for part of those poos making a second run through the gut.
Weighing on your kitchen scales helps you to get an up to date feedback about the food intake. If you switch from the normal weekly weigh-in and body onceover check to weighing daily if you have concerns or during illness/recovery with feeding support, please weigh first thing in the morning for best day to day comparison since the weight swings around 30g over the course of a day. Weighing first thing in the morning also helps you plan the support feeding level for the coming day as to the necessary frequency of feeds.
The poo output is always running late so you cannot control any weight loss through it but the size, form and composition can tell you what may have happened - mild or more serious tummy upset, diminished food intake and potential dehydration, reaction to antibiotics etc. Any soft poos are in the mild tummy upset category.
Weight - Monitoring and Management
Digestive Disorders: Not Eating - Diarrhea - Bloat - GI Stasis (No Gut Movement)