A question about my dog!?

ok, heres the deal my dog has always had diarrhea, and she has been to the vets, her stool has been tested, she has been on course after course of antibotics, the gator aid, the pepto bismol, but, still she has persinst diarhee ranging from semi hard to runny, she has a hearty appetite and dose not act sickley but she dose look sickley around the eyes, and everything she eats runs right thru her, her rectum stays inflamed also, and its definteley not a virus because this has went on for over 8 months and many vet vists, anybody have any ideals? your help is appreciated!