Ringworm does not directly cause diarrhea. Stress and anxiety can lead to diarrhea. Your pet may have developed diarrhea if it is very uncomfortable or stressed by the ringworm. Seek veterinary medical attention to treat ringworm and diarrhea.
No, it's a topical fungus. You should see your vet. If it wasn't diagnosed properly it may not even be ringworm or the ringworm could have been caught because of an underlying problem affecting the immune system.