What Causes Sporadic Bleeding And Alternate Between Diarrhea And Constipation?

I had a sigmoid colectomy a year ago due to diverticulitis. Since then, I still have sporadic bleeding, alternate between constipation and diarrhea, abdominal pain, nausea, and weight loss. I have had at least 3 colonoscopies and edg that show the diverticula has returned but not a cause for symptoms. I had a capsule colonoscopy that showed some dull areas but nothing alarming. I am scheduled for a doppler next month. I understand this is for lack of circulation to the blood vessels, but what does this prove? I have tested negative for chrohns and celiac. I am currently taking Vitamin D and B 12 due to malnorishment and plaquenil for joint pain and swelling (currently undiagnosed autoimmune disease). Any ideas?