Gastritis may be one of the reasons. Constipation can be other reason. There may be other possibilities.
What other problems are u having? Nausea vomiting sensation, stools? Any burning sensation. Any loose stools. Stress level?
Next Steps
have green vegetables as more than 70% of diet. many gi related problem will be solved if organic diseases are not there.
Health Tips
usg abdomen or some other tests may be required.