Hello, my name isXXXXX and I have over 20 years of experience as a veterinarian.
I'm sorry to hear that Lele is having trouble passing stools after her spay surgery.
Most of the time the first stool after surgery is the most difficult to pass, but we rarely see dogs having as much trouble as your girl is.
When dogs are anesthetized one side effect of the drugs is slowing down the gastrointestinal tract. Because they also don't eat or drink for a period of time longer then usual that also slows down the gastrointestinal tract and may lead to ingesta sitting in the intestinal tract longer then usual, and that can cause it to dry out and become firmer then usual.
When we spay a dog we incise through the abdominal wall. When she pushes to pass stool that is uncomfortable on those healing muscles so she may not push as hard as usual or stop pushing entirely and that stool will sit in the colon longer then usual and dry out and/or become larger then usual.
Finally some pain medications we use postoperatively, namely Tramadol, can lead to constipation.
With all of that it is amazing we don't see dogs having more trouble but we usually don't.
I'm glad to see that she is taking a stool softener.
If she is still eating well that is a good sign. I would recommend adding 1 to 2 tablespoons of canned pumpkin (not pie filling, just pumpkin) to each meal. It will increase the fiber, which stimulates normal gastrointestinal motility and is high in fluids which will help to make her stools softer.
I would also add low salt beef or chicken broth to her meals. The more fluids she takes in the better.
She should pass a stool in the next 24 to 48 hours.
Signs that she needs help sooner are complete lack of appetite, vomiting, fever, or a tense, painful abdomen.
Best of luck with your little one, please let me know if you have any further questions.