Hi,
Thanks for posting your query at DoctorSpring.com. I am Dr. Kini and I am pleased to assist you.
Recent onset diarrhea with abdominal pain is probably be due to a stomach bug. A stool test may be done for confirmation. Take yogurt, probiotics and drink enough fluids to keep yourself well hydrated.
The coating you see is probably mucus. Mucus is secreted by the inner ling of the bowel. When the bowel gets inflamed, there is increased secretion of mucus which might be what you saw as a coating.
I hope that answers your question. Please let me know if I can assist you further.
Regards,
Dr.R.K.