My Isa Brown has Diarrhea I think. What should I do?

Diarrhea shouldn't be a huge problem. Just make sure she is getting enough food, sometimes the poop can become watery because they aren't eating enough to make it solid.

 

Chickens have many types of droppings. Cecal droppings are slimy and runny, and occur with every 9th or 10th dropping. Drinking lots of water can make droppings runny. If you chickens are very young, then diarrhea or blood in droppings can be a sign of coccidiosis. How is your ISA brown acting? If she is lively, active, alert and eating well, she is probably fine. Probiotics which are found in plain yogurt and buttermilk, along with products such as Probios, are good to help in keeping the intestinal health of chickens in a good state. Use them only 2-3 times a week. Here is a link about coccidiosis and a couple about pictures of chicken droppings:
http://www.the-chicken-chick.com/2012/12/coccidiosis-what-backyard-chicken.html
http://www.the-chicken-chick.com/2012/02/whats-scoop-on-chicken-poop-digestive.html
http://chat.allotment-garden.org/?topic=17568.0