Poor little baby!
I have not personally had this problem, but a very close friend went through the exact same thing with her baby girl as you (diarrhea, blood in stool, etc). Turned out to be a milk and soy allergy- not to be confused with intolerance. The allergy went for so long undiagnosed by the dr (although she took her to the doctor repeatedly), that her stomach was horribly inflamed. Anything she ate went right through her and the stools typically had blood from the constant irritation. The GI specialist did little to help, and both the specialist and the ped said that she would grow out it. Finally, she got an appointment with an allergist, who could give her treatment options. After the visit to the allergist, her symptoms began to subside and she finally started improving.
One thing my friend did that you might want to consider is keeping a journal of how often the diarrhea happens, when the stools are bloody, and maybe even when she has been feeding and how much. If you are nursing, keep a log of what YOU are eating also. It will help tremendously as you go to all these doctors to have a log to present. Doctors are so quick sometimes to write off problems with babies as normal or "she will outgrow it". You know if something is wrong with your baby, so be persistent until someone helps!