Good evening, I'm Dr. Sara. I'm a licensed veterinarian who works exclusively with dogs and cats. I'm sorry to hear that Rixie hasn't been feeling well for the past few days :(
I'm glad to hear that her latest set of X-rays was unremarkable and I'm also glad that the barium is started so that they can potentially rule out a GI obstruction with her next set of X-rays. It's not unusual at all for dogs to strain to poop when they have diarrhea or colitis - this is akin to us sitting in the bathroom and feeling unable to leave, just in case we have to go again. They certainly can get stomach cramping when they have a GI upset too, and some dogs can look downright miserable. With any luck, this is a GI upset that will pass with supportive care and time, just like the majority of GI upsets in people. It could have been triggered by something infectious (think stomach flu) or something that she ate that was out of the normal routine. In either of those cases, some time and TLC and supportive care will have her right as rain.
The only thing that worries me from your description is labored breathing. It's one thing to breathe a little "different" from being uncomfortable, but if she truly seems labored, then I'd also check some chest X-rays too. I assume she is still in the hospital? If she's at home with you and she's having trouble breathing, this could potentially be an emergency situation, so I'd consider getting her to an ER.
If your vet hasn't done any blood work yet, this may be the next thing that they suggest if she's not improving with symptomatic care. Please let me know what other questions I can answer for you - we can chat until you're satisfied :)