Endometriosis Support Group
Endometriosis is a common medical condition where the tissue lining the uterus is found outside of the uterus, typically affecting other organs in the pelvis. The condition can lead to serious health problems, primarily pain and infertility. A major symptom of endometriosis is pain, mostly in the lower abdomen, lower back, and pelvic area.

As far as having pain flares in your stomach with the endo, I would certain to say there is endo in the stomach. Or perhaps the stress your body goes through when the pain comes is making your gastritis act up as stress can be a cause.
I would definitely ask your doctor about this.
I hope you feel better soon!
Stay Gold!