Antepsin and Zitac are often used in combination to treat gastritis in dogs. The Zitac acts to reduce the gastric acid production and the Antepsin works by producing a protective layer over the stomach lining. They are both well tested compounds that can safely be used in puppies. Screaming when trying to pass a stool will certainly not be related to either of these drugs, However it may well be indirectly related to the stomach upset that had required her to be treated.
The pain could result from one of two reasons. Firstly she may be a little constipated. If the stool she had first passed seemed a little hard this is a likely reason. If that first stool was soft or loose then it may be that she has strained on an empty bowel because it feels inflamed. So she feels something is there but there actually isn’t so the straining is unproductive and hurts her. Puppies are very sensitive to pain and can react quite vocally even though it may not be a serious problem.
If the straining/crying has now stopped (after perhaps 15 minutes) I would give her more time and continue with the vet’s instructions. If you think she is a little constipated it would not hurt to apply a little Vaseline (or a water soluble human lubricant) to her bowel opening so that it might assist her to pass a motion.
Watch for diarrhea and contact your vet if this happens. I would certainly get her back down to your vet if she is still crying after 30 minutes and he/she will be able to check to see if there is any obstruction.
Don’t worry too much, it will not be serious, but it needs to be watched, we don’t want her pain to continue.
Please let me know how she gets on.