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There are videos showing many other methods at the end of this article if you scroll to the bottom. You may see another one that seems more appropriate to you.
It will not hurt if she has to wait till tomorrow, though the whining is hard to endure when you want to help. It is a common situation. Somehow dogs survive our learning curve, as they also learn how to help you help them. It will be best to have her on bedding that will not become soaked in case she leaks. Don't worry, it usually takes a number of days of inadequate emptying before you have to worry about infection. That does not happen overnight. Please post back how it goes.
After that call, she pooped. Then she peed (without any help, in the bed) again and it's red. I've called the hospital to let them know, I'm sure the surgeon will call soon.
Worried about bladder rupture, vs. UTI (which I don't think flagyl is going to cover), vs. ????
I believe you when you say she seems to be in pain but I don't know where it would be coming from. She's at least 10 weeks post surgery, I would expect her surgical area to be pretty pain free by now. Dogs (as far as I know) do not usually stress over the return of sensation in the bladder.
It could just be general stress. That could also account for the gas in the stomach. Something to be aware of with larger dogs is the possibility of bloat. If enough gas builds up in the stomach, it can twist on the axis inside the abdomen and cut off circulation. Emergency surgery is then needed to tack the stomach in place. The surgery is a common one as this is not an uncommon condition with larger deep chested dogs. You might ask the vet is there is anything you can give her for the gas. Also ask if any of the meds she is on could cause gas. Hopefully the pooping she did will help with the gas. If some kind of bacterial overgrowth is causing the gas, the flagyl will help and flagyl works pretty quickly.
In the past when I have heard an owner/caregiver sounding or feeling very concerned about what is going on with their dog, they are usually right, so I would just say trust your instincts.