Cat is crouching low to ground when walking

Hi. I have a 12 year old male cat named Curtis. Curtis has urinary tract problems (crystals in his urethra when he was 6 and when he was 10), he had an upper respiratory infection in February of this year and we've had a new kitten since February of this year. The kitten constantly wants to exert her dominace over Curtis and will chase him and climb on his back and bite his neck. As a result, Curtis is always hiding under the coffee table, on the bed or under a chair. Since the respiratory infection (which he caught from the kitten), he seems to be a less happy cat and has not been his old self. Here is my question: last night he started slouching low to the ground when walking. This morning and all through today he has been fine. Tonight he began slouching again. He usually always responds to his name, especially when I say it, but tonight he seemed not to hear me and wouldn't look at me even when I was sitting two feet in front of him. He's not repsonding to being pet (not purring, not pushing his head into my hand, etc.) and seems a little weak. This went on for almost an hour, before I could get him to lay down on the bed and now he is sleeping and seems fine. Do you think that he could have a urinary blockage again (the symptoms were completely different before), or maybe another health problem that I don't know about? I have had Curtis since he was 6 weeks old and have never seen him behave this way before. Is he just getting old?? Has anyone else ever experienced this?

At his age, and with his history, Curtis should see a vet ASAP. Sounds like he might have a fever, hard to tell, but don't delay. Better to be safe than sorry.

Also, it sounds as if he really needs a safe and quiet place, away from the kitten. Stress definitely contributes to illness in cats. If he could have his own space, even if it's only one room, for at least part of the day, where he could get a little attention and snooze without being ambushed, it would help.

I agree with Badger,Curtis is probably very unhappy about the intrusion of a new kitty into his life,cats do not like changes in their routine and you should make sure he's not constantly harassed,especially with his medical history.
I would give the new kitten some time out on his own and spend some quality time with Curtis..but also make sure he has a place where he can feel safe.
12 yrs in a cat is not old,merely middleage and he should still be energetic,the crouching is probably from fear,but I am certainly no expert..
A visit to the vet,will answer all your questions and if he's found to be healthy,you can go from there...Good Luck!

Thank you Badger and Chico2! Badger - I think you are right about the fever, as I forgot to mention in the message that Curtis' ears were warm last night. He seems fine again today and is even running and bouncing around, so I'm not sure why his 'episodes' are only happening at night . . Livia does bother him more at night though so that does make sense. I will see what I can do about keeping them apart (small apartment) and will try to get him into the vet tomorrow as ours is closed on the holiday. Thanks again for your advice!