Hi and welcome
Please start syringe feeding any guinea pig asap that is not eating! It is abolutely vital to keep it alive.
You can use mushed up pellets in an emergency or ask your vet for herbivore recovery formula until you can order it online. Prep the syringe tip as shown in our guide to allow the fibre to come through. Weigh daily at the same time to control the food intake. You need to feed the more often round the clock the less your piggy is eating. Our guide explains in detail how much and how you feed depending on the situation and the independent food intake.
Not Eating And The Importance Of Syringe Feeding
Complete Syringe Feeding Guide
Once to start giving the antibiotic, please also give a pinch of probiotic either 1 hour before (US recommendation) or 1-2 hours after the antibiotic (UK recommendation) to bolster the guts.
Please see your vet promptly if your guinea pig takes a turn for the worse or if things do not get any better after 3 days. Ask for a scan this time so you and your vet can get to the bottom of what is going on.