Gastritis is an inflammation of the stomach. In this case you'd feel pain in the middle upper abdomen, just below the breastbone. You'd probably have heartburn. Pain would be worse after acidic food (vinegar, vitamin C, aspirin...)
Upper right pain is most often from gallbladder (stones). Pain would be worse after the meal, especially one rich with fats. CT cannot exclude gallstones, since very small gallstones can not be detected. Ultrasound is also not enough.
You can:
1. observe, if symptoms is worse after a fatty meal (this speks for gallstones). If yes...
2. ...then you can have HIDA scan - this is the most accurate test for gallbladder function.
What meds are you taking?
Could be your gall bladder, it takes some drs. forever to admit that's what it is.