Diarreah in rabbits is NOT good. Here is some helpful info from http://www.snow-berry.com:
This can be an emergency situation. Rabbits do not live long if left with diarreah. Young rabbits are especially vulnerable. They can die in as little as 24 hours, going from a perfectly healthy rabbit with normal droppings to a dead rabbit by morning.
Prevention is important. I do not recommend feeding greens and vegetables to a rabbit under 4 months old. If you choose to feed veggies, then introduce slowly in small amounts. Feeding fibrous timothy hay is recommended. Oats (Old fashioned rolled or flatted horse oats) make a good treat.
Young bunnies sometimes get droppings stuck to their bottoms, but these are sometimes cecotropes (the morning droppings that they eat). I usually trim the fluff from their bottoms to prevent this.
If your bunny is suddenly listless, sitting in the corner, bloated or sloshy sounding belly, possibly grinding his teeth, and has a very wet tail and back feet, he may have Enteritis. He may have clearish or greenish jelly poops, which may have a different smell than usual, you need to act quickly. You might notice that he's stopped eating and drinking as well.
Fluids are vitally important at this stage as dehydration kiills. I will have a 5 CC syringe handy to drip water into the rabbit's cheek. I put him on his back, and drip slowly so that he doesn't choke. I keep flipping him rightside up so that he can swallow. I usually mix a vitamin/electrolyte mix into this water.
After the fluids, I make up a mixture of Pepto Bismol (I used 1/4 of the tablet crushed- not sure of how much to use of the liquid). I also mix in an Acidophilus/Bifidus capsule of powder to hopefully help restore the gut bacteria. I'm not sure if this works, but it doesn't hurt them. Finally, I crush up a 1/2 tablet of digestive enzymes (pineapple and papaya enzymes). I make a watery paste with this and syring this into the bunnies cheek. I try to get as much in as I can. I do this morning and night, until I see a change. Keep up with the water throughout the day!
If you are stuck for the above ingredients, another thing to try is to feed fresh cecotropes from a healthy rabbit to the sick one. These look like long grapelike clusters, and are different from the fecal pellets that look round like marbles. These are usually eaten by the rabbit right from the anus, and provide the rabbit with special bacteria and nutrients.