It is possible to experience not only diarrhea, but also symptoms like nausea or constipation while on gabapentin. Some people also report heartburn as a side effect. Unless the gastrointestinal symptoms become severe, doctors will often recommend changes in diet to combat them while on gabapentin.
Gabapentin has also been associated with a number or unwanted effects which affect the blood, pancreas and liver. Persistent stomach pain with sickness could be a symptom of an inflamed pancreas. A skin rash or swelling on the mouth or face could be a symptom of an allergic reaction.https://www.nhs.uk/medicines/gabapentin/