I agree i think my here is something in her new food that does not agree with her i would check the ingredients of both dog food see if there is something different in the new one that not in the old.I would take her to the vet or give them a call.Hope she better soon.
Diarrhea isn't normal, no. Whether it's par for the course after treatment for a parasite or not, I am not sure. I've never experienced that problem.
Change of food can cause diarrhea, but like you said, if it's done gradually, usually there's no problem. Unless the new food really doesn't suit her...
I would take her in. Let the vet check her out. Don't know if it will be possible to take a stool sample?? Try. It might help with diagnosis (rather you than me! lol!)
I hope she is getting better soon.