Two of my 4 cats have consistent diarrhea and they are starting to get really sick. HELP!!!!!!!!!

About a year ago I got a cat named Bubbles. She had diarrhea before I got her and no one at the shelter knew why. I got her tested for everything under the sun and she came back negative.  The vet said she has IBS and needed to change foods. After a few months of giving her very special food and it not changing anything, I took her back to the vet and they got a fecal sample. The result came back indicating that she had some bacterial infection and they gave me antibiotics and steroids. That worked for the duration of her medication but a week after her dose ended her diarrhea came back.  This cycle went on for months- giving her antibiotics when her diarrhea got bad and it working, then it coming right back. During this time, she acted completely normal in every way and played with my other two cats who have no symptoms. A few months ago, I got another kitten named Fiona. She was perfect and healthy and had no problems other than fleas. A month or so ago I noticed she is having diarrhea too. She was reaching her developmental milestones on time until just recently and now she weighs just 3lbs when she should weigh 6lbs, but otherwise seems healthy. I took her to the vet and they did a smear test and everything came back negative. The vet gave me antibiotics for both Bubbles and Fiona and the week is up and Fiona's and Bubble's diarrhea is still very prominent. Bubble's has had diarrhea non-stop for a year and now Fiona has had it for a month and I have tried everything under the sun and have no more money. I was never worried about it until now:

Just recently

=Bubbles will all of a sudden hiss and meow and act like she is in great pain while licking her butt. These episodes last 30 seconds and then she is 100% normal again (playing, eating). These episodes occur once a day at most.

-Fiona has a very swollen butt-hole and is grooming it way too much. She has gained no weight since she was 3 months old and now her anus is black. She is a black cat, but I could have sworn it was all pink/red just last week and now it is black around the edge and pink in the middle. 

More info:

-I have 4 cats, only Bubbles and Fiona have any problems.

-Antibiotics won't help

-both seem playful and perfect other than what was mentioned

-worms came back negative

-haven't done a fecal sample for either, only fecal smear test 

-have tried, antibiotics, special food, medicine to add into food everytime they eat, steroids,

-FIP and FeLV negative          

-Bubble's and Fiona poop is yellowish-brown and runny, but not watery. It has some shape to it.      

-Gum color and everything is normal          

-Fiona seems playful, but sometimes she seems more lethargic than usual, could be my imagination?

-Everytime I've taken them to the vet their temp. has been normal. 

*Fiona's parents were both unusually tiny, could be reason for her small size, but I doubt it. 

Please help!!! The vet hasn't helped me and it has been over a year and my cats are slowly dying. Idk what else to try

My cat had consistent diarrhea.... long story short (and after $$$ spent at the vet with no relief), I discovered that when I fed him food without gluten (or wheat gluten) in the ingredients, the diarrhea stopped immediately. You must read labels carefully, but it seems to be the ones with the gravy have gluten in them. I use the Fancy Feast "Classic" only - for all my cats.