Is it advisable to have a colonoscopy while rectum is inflamed?

I can't say one way or the other in your case, you should really talk to your GI about it, but I know from my experience it was not a good idea. I had one years ago with active inflammation especially in the rectum and an active perianal fistula. Most miserable experience afterwards dealing with a terrible temp and painful fistula. It just made everything worse. I stopped going to that doctor because of that experience and was told by a number of doctors after that already knowing I had very active crohns and the fistula before the colonoscopy put me at a high risk for sepsis and never should have been done. My last GI didn't even want to do a colonoscopy knowing I had had bad rectal inflammation in the past (which didn't do much to help me figure out whether my meds were really working or not).

My symptoms actually improved after doing the prep. Getting rid of the fecal material helped calm the immune system for a short period.

The prep can cause dehydration. You want to be sure to drink a lot of water to prevent that. Otherwise you'll get fever/chills symptoms.

If the colonoscopy reveals any areas of concern, you'll likely need an MRI next.

If your condition is severe and not controlled, you should be getting colonoscopies frequently (like every year or two) to make sure you're not developing fistula's, abscesses, polyps, cancer, etc.

Always work with the best doctor your insurance will allow.