It's extremely hard finding any place that will allow me to live with a cat, let alone a cat that is marking territory inside the apartment. I need to nip this quick before I either lose my apartment or my cat!

A couple of months ago, Squish got a really bad injury from being an outdoor/indoor cat, I decided to keep him indoors after that, especially seeing as he's not the biggest male cat on the block, but is almost always the 1st to start fights. I recently had to run across town twice a day for 3 days straight to take care of another cat for a friend while she was out of town, and of course I got a generous helping of cuddle time in with that cat. The 2nd day of taking care of the other cat, Squish marked a rug on the floor, the 3rd day, he marked my sheets and my pillow on my bed. He's so territorial, that whenever he sees another male cat outside, even if that cat is like 20 feet away, he'll claw at the door like he was trying to escape a fire and scream at the top of his lungs and start breathing really loudly. It's like he wasn't even neutered in the 1st place! But he hasn't marked anywhere in the apartment for the past 7-8 years he's been alive until now.

He doesn't seem to have any trouble using his litter box, no struggling whatsoever and his urine doesn't smell any different than it did previously, so I don't think he has a UTI. I don't know what to do, between the severe allergic reactions to the fleas I can't seem to get rid of and now this, I am starting to wonder if we should part ways. Ugghh, that sounds awful and I hate thinking these thoughts, but I don't know what else I should do. I can't have him marking everywhere any time I pet another cat.