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Assuming your piggie is full grown, a piggie suddenly producing much smaller poops than normal is a first sign that something may be wrong and should be taken very seriously and treated quickly (unless it is a recently neutered boar who tend to produce smaller, rounder poos after the op than before!)
Best to take her to the vet tomorrow to get her checked out.
How much does she weigh and his she losing weight? You will need to weigh her daily to watch for signs of any weight loss.
Small poos suggests that not enough food is going through her system and coming out the other end. There could be many reasons for this but a depressed apetite and decreased gut mobility could be an indication of dental problems, pain from another condition such as bladder stones/urine infection, respiratory infection or something else.
If the cause isn;t dealt with quickly and the piggie kept fed, then the gut can go into stasis which can be fatal for a guinea pig.
If a piggie isn;t eating properly I would normally suggest supplementing with Critical Care/Supreme Recovery (or mushed pellets) via syringe to keep her weight up and get her gut moving again properly.....however as there is no indication as to what is wrong you really need the vet to rule out an obstruction in the gut before you start feeding her....hence why you need to see the vet pronto if she isn;t eating.
f you can observe her closely and provide answers to the following questions to your vet it would help with a diagnosis
Are the poos tapered towards one end, strung together, dryer/squishier than normal or have bits of brown mucus attached?
Is she eating and drinking normally - hoovering up all her veggies and pellets and also eating lots of hay? Or is she being selective about what she eats? Is she able to eat the veggies properly or does she let bits drop out of her mouth or chew carrot up into tiny pieces?
Is she weeing OK? Is there any discolouration/smell to the wee?
Can you hear any respiratory sounds when you liten to her chest - is there any sign of snuffly nose or crusty eyes?
Good luck tomorrow - I'm sure there;s a simple explanation for the sudden change in poo size that is easily treatable