My Stomach Has Been Hirting Alot Lately Especially After I

My stomach has been hirting alot lately especially after i eat. I thought i may be just constipated even though i have went a little bit over the past week. I took an enema one day and had some results but after eating i started to feel bad again. Even causing me not ro be able to sleep. I took some mag citrate and after a couple of hours had resilts but patcjes of the stool was black. I have had a gastric bypass and was worried about the color. Should i be concerned