Passed light brown tissue with mild to severe abdominal pain.

I'm 23 yrs old and for about a week I've had pain on my left side varying from mild to severe with extreme constipation. I was having diarrhea before the pain started, so I took an immodium. My doctor said he's pretty sure it's colitis, since the pain gets worse after I eat with gradual improvement if I don't eat for a while. I'm still having the same pain, but I just started having small, firm bowel movements. All of this was fine, but today I passed some kind of light brown tissue. I'm fairly sure it's not mucus, and certain it's not bloody at all. Very confused. It just didn't feel right. Any thoughts?