Went to the er with a lot of blood in my diarrhea. they did a ct scan w/ contrast. there is mild diffuse wall thickening of the descending colon and parts of the sigmoid colon. also noted was a umbilical hernia 1.3cm. what could cause the blood ?

Inflammation: The findings you describe is what inflammation or infection may show up on a CT scan. There are may causes of these findings that could be cause abdominal pain, blood and mucous in the stool, and diarrhea for example. To complete the workup, you need a colonoscopy for possible biopsy and and the colon can be visualized. Treatment then can be started. I doubt the hernia is a significant problem