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I am sorry your guinea pig is unwell. Digestive upset is common in these types of animal and they can deteriorate quickly, so if improvement isn't being seen you should pop to your vets as more invasive management or medications may be needed.
The important thing at the moment is to encourage eating and get any impactions of food or gas moving through. So offer fresh herbs like coriander or parsley, or vegetable puree (baby food is good) and also fluids should be going in well too, if not pineapple juice is a great way to get sugars and fluid into a guinea pig. This can be offered like normal food and water but if not being eaten then can be carefully syringed into the mouth. As I mentioned before if improvement isn't seen or things worsen you will need to go to your vets. It may not be an impaction of food or gas and may be something else causing the abdomen to look bigger.
Good luck and best wishes