loose and leaky stool

We've had our calico cat Callie for about a year and a half now. I picked her up out of a parking lot on Christmas eve; by the vet's estimation she was about 2 at the time. Since we have had her she has always had loose and very smelly stool. I put her on a diet of Science Diet Sensitive Stomach. It doesn't seem to have done much. But within the last week, she has started having poop seep out of her butt while she is walking around/sitting/sleeping... etc. She has no changes in mood, the amount she eats or sleeps, no changes.. just leaking poop. For the past week we have been feeding her Royal Canin Digest Sensitive wet food mixed with Metamucil and organic canned pumpkin for fiber (per a google search result for what to do about this problem). It seems like there may be a very slight change for the better based on the amount of times per day we have had to clean her off, but it is by no means fixed. She has been checked for worms at least twice now over the course of the time we have had her and both were negative. I think next we will try boiled chicken diet. Just wondering if anyone might have any ideas as to what this is??

Is it possible she could have a problem controlling her anal muscles? I don't really know how this could be tested, what's her urination like? can she controll that.

What about her bum in general is it swollen or puffy, any parts that look inflamed?

does she try to use the litterbox? 

I'm thinking if she doesn't really try to pass stools and it's seeping out it might be that she can't feel she needs to go.

I guess it is possible that she can't control her muscles, but I don't know what that would mean health-wise. Her urination is fine, she does that in the litter box. She poops in the litter box too when she has to. She just comes out with some still on her that has to be cleaned off and it just comes out periodically no matter what she is doing.
She doesn't really appear to be swollen or inflamed. During the past week it has been inflamed, but I figured that was because we were having to clean her off so often.

The inflammation could be due to the cleaning, depending on how often and what you are using. If she is using the box then she more than likely can feel when she needs to go.

Maybe the problem is with the digestion it could be that the digestive tract like part of the intestine or colon isn't managing to absorb the moisture out of the feaces all the time, is she seeming deyhdrated or anything?

Again I don't really know how they would check this in a cat without invasive testing.

I'm sorry I haven't got any difinitive information on this subject, I'm more or less just thinking about things that cause leakage in humans. Since you're giving her a specialised diet and it's ongoing it doesn't seem to be dietry.

No, she's got something going on in her, I would highly suspect a parasite like coccidia.

If she hads not had a fecal float done then I would do this.

Even then sometimes the cysts are hidden and not being shed at the time and show up as negative, but there is another test using a microscope that WILL find the little nasty intruders.

Some vets will treat for parasites despite the neg result.

I would almost guarantee this is the prob.

If it's that runny it can leak out a bit, I have never seen a cat with no control, that's a good, strong muscle there, but will still leak if there is liquidy poop inside.