Colitis (without diarrhea), constipation, and Flagyl?

Hi everyone!  My 15yo girl was recently diagnosed (3 weeks ago) with her first case of colitis without diarrhea.  

A little background: She's been eating Wellness grain-free canned food for the past 13yrs and I always mix in 1-2tbsp of water with her meals to ensure adequate water intake.  She has a history of pancreatitis attacks and her most recent attack was 3 months ago. We've tested her fPLI and it was elevated, even when she didn't have pancreatitis symptoms.  Her appetite is healthy, she seems to be feeling OK.  Her last bloodwork, performed 2 months ago, was healthy.

After xrays were performed and reviewed by a radiologist, no constipation was found. The vet suggested I incorporated pumpkin into her diet since even though she's having her normal bowel movements (every other day), they're slightly on the drier side.  Each time after she has a normal poo, she runs back and forth to the litter box trying to have another bowel movement. She then starts squatting everywhere and recently I began seeing some mucous when she was trying to defecate.  

My vet was bewildered, so I consulted with another vet clinic (I didn't take her with me, just her records and xrays).  After hearing about the straining post defecating, the vet said it sounded like colitis.  He explained that perhaps she was mildly constipated before, and when she pushed the harder stool out, it may have irritated the sensitive lining of the intestine (hence the increased mucous production). If it's inflamed, it's going to get re-injured each time she defecates (or pushes anything through).   He also said that it makes sense that she would have intestinal inflammation if she had chronic pancreatic inflammation (since they're usually linked along with the liver ("Triaditis")).  

She has a mild heart condition, so I have an appt on 11/17 with her cardiologist for her yearly exam.  I want to schedule the internist to do an abdominal ultrasound at the same time. I'll also have a full blood panel including the pancreatitis/GI profile (fPLI, fTLI, cobalamin, folate).  She needs to be fasted for the ultrasound and the GI profile, so I thought "why not just fast her 1 time and get it all over with?")

Even though she doesn't have diarrhea, the vet prescribed Flagyl, since it's an antibiotic with anti-inflammatory properties.  I provided a fecal sample on Saturday, so we should have results today/tomorrow indicating if there are any parasites/bacteria, in which case the Flagly would be warranted.

The vet prescribed 125mg 2x per day, but after reading all of the horrible side effects that could happen, I gave her half that dose (62.5mg).  She was completely out of it for about 13hrs.  I hated it and thought I might try to reduce it even further (31.75mg  2x per day = 62.5mg/day).  I actually haven't started yet, I'm waiting for the fecal results.  

My questions are:  

1) Does anyone have experience with colitis caused by constipation (not diarrhea)?  

2) Do you think that the 1/4 dose of Flagly (31.75mg) 2x per day would even be effective?  

3) If there is no bacteria or parasites, is there another (safer) anti-inflammatory that I can give her, since the antibiotic wouldn't really be warranted?

Thank you in advance for your replies!

I can't help with this, but am bumping up your thread so that others can see it.

My kitty has IBD which always included constipation when he had a flare.  He is now on prednisolone to control the inflammation.  He had his first flare when he was only 18 months old and his symptoms mimicked foreign body ingestion, constipation, anorexia and vomiting bile.  I went through several vets before finding a vet who diagnosed IBD and started the steroid.  The other vets always included Flagyl as part of supportive care when he was sick.  I don't know if it helped or not but the only issue that he ever had while taking it was the horrible taste.