Here are some reasons why a guinea pig's stomach might be beating on one side, but please remember that only a vet can diagnose and treat your guinea pig's specific situation.
* Bloating: This can be caused by gas buildup or other digestive issues. It's a serious condition that needs immediate veterinary attention.
* Internal organ problems: Other internal problems like a tumor or infection could also be causing this symptom.
* Respiratory issues: Sometimes a guinea pig with respiratory problems might have an abnormal heartbeat or a beating sensation in the chest area.
Please don't hesitate to contact a veterinarian right away. They will be able to properly assess your guinea pig's condition and provide the necessary care. Good luck!