Causes

A rabbit's good gut bacteria can get out of whack due to stress, change in feed, too much of a good treat (like carrots), a dirty cage or dishes, coming in contact with a sick animal like a mouse, or a family member came in contact with a sick animal and then visits bunny.

First assess what caused it.
Did you fed too much of a good thing or introduce a new treat like yogurt treats, greens or carrots?
Did you switched the brand of food?
Or is it a bacteria infection? 

Rabbits have several kinds of bacteria living in their digestive track ( so do all animals).
A good kind that helps them to break down food and absorb it
and some bad kinds like E-coli that all living animals have.
When something happens to cause the food to ferment in the rabbits stomach the bad kind of bacteria level become too high and will make the rabbit very sick. 
It becomes very difficult to get it under control since any medication you give a sick rabbit to kill the bad bacteria will also kill off the good kind. Good bacteria not only break down food so that it can be digested but good bacteria create an environment in the intestines that makes it difficult for the bad bacteria to grow. 

If your rabbit has extreme diarrhea that doesn't want to stop, in most cases the rabbit won't live.
It is always advisable to take the bunny to a vet as soon as possible. 

If you are unable get bunny to a vet then below is what I'd do. I know my vets don't seem to have an emergency, they book everything 12 hours from the time I call and that is just too long for a rabbit. 

Try to get bunny to a vet to see if it is bacterial.  The vet can do a culture sample and prescribe the best drug to kill an infection. Usually a sulfa based drug.

Treatment suggestions

- Start by feeding him good grass or timothy hay (alfalfa hay is too rich and can encourage the wrong bacteria. 

 -Force feed Critical care or rabbit pellets & a bit of water in the blender. (see my first aid kit page)
Use a feeding syringe or if don't have one then a pastry bag and tip used to ice cakes.
You need to get the right food in the rabbits gut again. You can buy Critical Care over the counter at most veterinaries for approx $22.00 for a tiny bag but it lasts a very long time.  You want to get some good high fiber hay grass or timothy or pellets in your rabbit as soon as possible.

 -Feed Ben-bac or Pro-bac to replace good bacteria. (see my first aid kit page) 

  -Stop the diarrhea: an eye-dropper of Pepto-Bismol or any human diarrhea medicine.

-Make sure your rabbit gets some water into him, you can put a little bit of sugar in the water to encourage bunny to drink. Diarrhea dehydrates animals. Often to the point were their blood pressure drops so low that a rabbit can die from that. Low blood pressure means low body temperature, shock and/or heart failure. You can wrap bunny in a towel a feed a bit of water via a needless syringe. A little bit at a  time. This is a very slow method. It can't be rushed or your rabbit will get water up his nose and in his lungs. 

- A vet can give the bunny a Sub-Q (water under the skin and they can show you how to do this. I've had to learn and I really don't like needles but I've saved a few bunnies. It is a much easier method for  you and the rabbit then the syringe method. The rabbit can feel a lot better within minutes.

  -give a little bit of Human anti-gas medicine like gas-X.
If you manage to stop the diarrhea, your rabbit will likely have constipation and gas trouble. 
This is due to the lose of water, food and the good bacteria. 
Gas is very painful for rabbits and they will refuse to eat. A rabbit that doesn't eat or drink can die from pain or liver and kidney failure.

 
-If bunny feels cold from low blood pressure than you can  lay him on a hot water bottle or heating pad. Make sure you get water into him. 

Giving a rabbit a Sub-Q (water under the skin)

 -I'd give my rabbit sub-Q fluid by grabbing the rabbit by the skin between the shoulder blades and pulling up (like a mother cat carrying a kitten). 

I place the needle in the little tent of skin I have in my hand.

I pull back on the syringe to make sure no blood enters it. That would mean I hit a vain (almost impossible to do in this case) and then when no blood in seen I administer 50 to100 CCs of sub-Q (sterilized water) under the skin.

The rabbits body absorbs the water in minutes and it doesn't hurt the rabbit. I've noticed a huge change in my rabbit within minutes.

The Mystery of Rabbit Poop   
by Dana Krempels, Ph.D.

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