Help my sick pittie

You might want to consider that she has irritable bowel syndrome or inflammatory bowel syndrome.  Ask your vet for more information on these two conditions.  Your vet may prescribe some antibiotics and also may suggest a special diet to see if either helps.

I would definitely keep trying to find a diagnosis from a vet or an internal specialist. Losing that much weight despite eating a lot would concern me very much.  Good luck.

Yes, ditto to everything Jackie said!!  It may be time for you to "think outside of the box" a bit -- If your vet isn't open enough hours or isn't willing to just have you rush the dog in when it strikes then I'd be changing vets or I'd be considering going to a vet school (it may be a drive but often it can be cheaper than a specialist and more cutting edge).  

But in honesty -- you have to get her **somewhere** to be examined when she's having the problem.  There are some parasites that aren't easily seen, and they then may need to look further.  

Sometimes we have to kick it up a notch to MAKE SURE a vet is paying attention when we need them to, so honestly evaluating WHY it is so difficult to get this dog seen and fully evaluated when it is sick is a big deal.

They should be able to go inside with a rod to catch even a small sample from inside the intestines -- and you may need to leave the dog at the vet so they can try to monitor around the clock.

I agree with trying a new vet.  We actually took her to a new vet yesterday to get some new ideas.  They did try and take a fecal, but even with the scope, they couldn't get enough to make a diagnosis.  Thank you for the vet school idea.  I hate the idea of a bunch of students poking around my princess, but it may be the best way to get her a formal diagnosis.  The new vet does think it may be a small or large bowel issue.  We are also now working on a way to get her to gain weight.  She is only 46lbs.  They think she may be having issues absorbing her food correctly. 

I'm sorry to hear about your pittie's issues.  Not having a diagnosis when our pets are suffering is so upsetting.

I have no direct experience with chronic or acute intestinal issues in a dog, but our friends had a Basset Hound who had chronic diarrhea that could not be diagnosed as anything "usual" (not parasites, not food allergies, etc.).  They finally took her to a specialist who did some kind of procedure to biopsy her intestinal tract to look for possible cancer.  The diagnosis turned out to be something called Lymphangiectasia.  It's a condition that prevents proteins from being absorbed, or something like that.

I don't know if  this is something your vets have considered, but our friends' dog had a couple similar symptoms as yours:  chronic diarrhea and major weight loss, despite plenty of eating.  I hope this isn't what your dog has, but I know how hard it was for our friends before they finally got a diagnosis, so I thought I'd  suggest it, just in case it helps.

Sending good thoughts to you and your pup.