I’m curious why my cat has had diarrhea for more than a couple weeks

About 2 to 3 weeks ago I took in a stray that was abandoned at one of my jobs. I took her to the vet. Just for a routine checkup and he determined that she had feline herpes as well as a small upper respiratory infection. He prescribed her an antibiotic and a pink medicine that was supposed to coat her throat. I mention to the doctor that she was unable to meow and that whenever she opened her mouth no sound came out and he explained that it could be a cause of the long-term feline herpes that she had and he was unsure how far down in her intestines the herpes have gone. Well, after finishing her medicines she is on the same diet since the day I brought her home and she still has constant diarrhea. Though she acts like she is feeling fine. I’m very curious on what could be causing this and if I should bring her into the vet and use my time and money on a physical exam or if I should just bring a stool sample into my vet and have them examine that just to cut straight to the chase. I appreciate your time and advise and I thank you for what you do with all of these animals may God bless you and all of those around you.