Concerned about my older cat

Hey friends, thank you for taking the time to read this thread and possibly help me out.

My little kitty cat is 15years old and still a bundle of love and energy. She's extremely attached to me, despite me only being her owner for a few years. Demands to sleep under the the covers with me AND be cuddled or else she throws a fit. I wake up to a paw in my face or my hair getting pulled until she gets her snuggles again. I love her as much as a person can love their cat. She still loves to play, responds to any bird/animal on the T.V and runs all over the house. Sadly, I'm starting to have some issues with her. I'll never abandon her, since I consider her my best friend but I digress.

Her previous owner decided to give her to me because she was having some issues with her. She was almost 13years old and started pooping on the floor.  Since I'm in a relationship with this person, and have lived with her in the cat, I gladly stepped up to the challenge. It took me about two weeks of trying different things to get her to stop pooping on the floor. I found out that if I kept only some litter in one side of her litter box, then cleaned it daily, she would be fine. This worked great for a long while, but now she's at it again and I can't find a solution. I've been to the vet and they don't know what to do so I'm worried that I'm going to have to do rounds cleaning up her messes for the rest of our time together. The vet said that it possibly might be her way of rebelling, but that was his only guess. I've been trying to pay a lot of attention to her and let her sit on my lap 90% of the time I'm at home. As of right now, it's still happening.  Then the other night I noticed something odd that she's been doing for a while. Every once in a while, especially when she just wakes up from a nap, she does this odd little three rapid short inhales or exhales. Imagine just tapping the nozzle on a tiny air compressor 3 times, and that's the sound. Now, some people might call me stupid for not actually paying attention to this sooner, but the vet said she was in good health! I'm worried that she might be ill. Having heart or breathing issues.

I'm sorry I wrote such a long story for you guy, and I appreciate any feedback/advice.  I know a lot of what I wrote was probably didn't need to be included, so I apologize again.  If anyone has any idea or direction to point me in, please don't be afraid to chime in. I'm seriously at a loss as to what to do. I can't afford too many more trips to the vet before I have to start pawing half my darn house.

-Botis

I'm still trying to get my one cat to start using the regular litter box again since she doesn't want to leave my bedroom while the dog is out here So poopy all over - yay! Not to mention pee. I have to throw out half the stuff in my room.

One thing I'm considering trying is those pee pee pads they have for dogs. Maybe she just really doesn't like litter? Or does she have trouble getting into the litter box? Did ANYTHING in the house change recently? New piece of furniture, etc? Cats are really weird about little things like that, I've noticed.

As for the breathing thing, have you taken her to the vet since you noticed it? She could just have a cold. My Lucky had a cold for quite awhile it turned out (coughing a lot the last week before taking him in) but then after a course of antibiotics in July, he was fine.

Is there a pattern to where she's pooping? Oftentimes uncovered or poop out of the box is a territorial thing. I'm wondering if she's a little high-strung, anxious, stressed, etc so she's reacting by trying to reinforce her territorial boundaries. Have you tried Feliway?

Hey, thank you both for responding.  I was actually starting to wonder if maybe she just hates the texture of litter on her paws. I do recall her rubbing all over my laptop case when I brought it back home. My girlfriends cat was laying on it when I was visiting her, and her pooping on the floor again happened not too long after she discovered the laptop case with a different cats scent on it? I guess it's a possibility that this is all behavioral, which is sort of what the vet suggested in the first place. The sniffling is something that has happened in the past but only once in a blue moon and enough to make me raise an eyebrow. I did inform the vet the last time I was there, but I was told everything looked fine and blood work was normal. Recently I noticed that she's been doing it more often, so it has had me worried. I think I should take one more trip to the vet just to be extra extra safe. She's getting up there in age, so every little thing that seems off about her really starts to get me nervous.