It could be anything from a simple case of indigestion and bloating to Gall stone disease and appendicitis. Your 4 month long history suggests that it is a more chronic condition and may depend on a lot of factors like your sleep time, food habits, smoking and alcohol and h/o any other medical conditions.
Next Steps
You might need an Ultrasound whole abdomen to start with aided to clinical examination of your abdomen with an experienced doctor.
Health Tips
Try changing your diet and sleep habits, if you think there has been any changes recently that might be affecting your health.