Hi, unfortunately one of those "emergencies you must take your rabbit to the vet ASAP for" is baby bunny with diarrhea. I am assuming she is under a year old?
Young bunnies are susceptible to imbalances in the gut microorganisms, leading to enterotoxemia or coccidia. Coccidia needs to be treated with a particular drug that only vets have. The bacteria that cause enterotoxemia basically pour toxins into the bunny's system, which can cause the body to literally shut down, so she needs a vet to treat that as well.
If you go to a vet, and I really hope you do, make sure to bring a sample of the feces.
If you decide you still cannot go to a vet, take out all other foods except hay and water. Get some pedialyte and syringe it to her--about 20 mL should be good to start. Get some Bene-Bac (from Petco or Petsmart, etc), and give her 1 gram. Also, give simethicone (mylicon) 1mL/hr for the first hour. This is just a repetition of what angieluv said, but I wanted to drive the point home.
If you can't afford to go to the vet, ask if they will take Care Credit, which is a credit card for vet care--not the best solution, but it is a temporary solution at least. Another option is IMOM
http://www.imom.org/fa/
Finally, if you can't afford any of these things, consider taking her to the local humane society. They may be able to treat her cheaply and give her back to you, or they may not be able to give her back. You will have to ask. However, with something as serious as diarrhea in a young rabbit, it may be worth it to save her life.