yeah, my gut has me agreeing that its most likely a hemorrhoid and diet related. I realllllly hope its not the TF back again. I will keep you all posted with what the vets suggestion is
I agree that adding raw chicken should not cause runny stools. In fact adding raw meat is the only thing I don't think needs a gradual transition. Usually transitioning to a new diet you wish to do it gradually, yet when going to only raw it doesn't cause an upset...I guess because the only ingredient (meat) is not the problem in the other diets it's all the other stuff in the cans and kibble!
So saying that, how careful were you with the chicken thigh you did give him raw? Was it from the same batch of thighs that you had given him a cooked thigh from a few days ago? Had this raw thigh been sitting in the fridge for a few more days? If so, then I might be more careful with the raw chicken from now on. My rule is that if it doesn't smell good enough for me to cook and eat for myself, then I won't feed it raw to a cat. I'm really fussy about what I cook for myself too (I'm a microbiologist by training!)...
Chicken is probably worse than other meats too, it's not just Salmonella but the counts of Campylobacter are high from the processing of chickens in plants. When grinding chicken for Babbage (he's the only one eating chicken here because he's not allowed any bone in his diet and it's too hard to debone rabbit on a regular basis), I get a large batch from Costco, rinse the pieces before grinding...add supplements and oatmeal and fiber (he's got a special diet due to his digestive issues) and then package and refreeze. Then I thaw the next day and so on for his meals. One bag is usually enough for a day or two. The freeze and thaw cycle helps kill off some of the bacteria...not all but it does have a good effect on the type of bacteria you tend to find on chicken. So not something we like to do a lot with the meat we consume but good to do when feeding it raw to your pets.
I would take him to the vet, have his poop checked for infection. Obviously you want the runny poops resolved asap!
What my vet recommends as a diet for kittens and cats with diarrhea is boiled chicken and rice... some ground chicken boiled up with a lot of water and a little white rice added. This is very bland and tends to settle their tummies down best, then you can work to reintroduce the diet you've chosen.