Why Is My Stool Dark Or Black After Eating Mainly Rice, Beef, And Fried Fish For Three Days?

I have been eating just rice with beef and fry fish for last 3 days and also missed lunch and my stool is dark or black and I drink water very little as much as one glass a day. I also have GRED for last 6 months and gastric for last 5 years or more. I don’t feel major pain on my belly often. But when I do the pain seems to be moving around my chest area, sometimes even in lower back. And I’ve headaches at night for last few days. I’m worried why my stool is black or dark? Is it because of diet? Please suggest me what should I do to keep my gestational track healthy and get rid of gastric.