When I had my Bengal kitten, I had this problem with her. It happened when I tried to switch litter brands and she was also being brought into a new home. Suddenly she peed on the middle of my bed, soaking my mattress in urine. On top of that, she peed next to her litter box several times; apparently it wasn't clean enough, even though there was only one thing in the entire box.
She also peed on soft surfaces, such as doughnut beds, pillows, and even plastic left on the floor.
My solution was:
1. Immediately switch back to the litter the breeder had been using; i.e. her favorite kind of clumping litter.
2. Give her two large and yet low-sided boxes of her own, placed in her own room. This way she had one box to pee in and one to poop in.
3. I cleaned all boxes in the house twice a day.
4. I kept the door to my bedroom shut at all times. I never gave her the opportunity to go exploring anywhere near my mattress again!
5. I bought product to thoroughly clean the mattress.
6. I also took her into the vet for a check-up. It was not medical.
She started using her two boxes. She definitely was happy to have two litter boxes. I still did not trust her near my bedroom, because I wasn't convinced that she couldn't still smell where she had urinated on the mattress (it was before she had been spayed and her urine was very strong even though she was only four months old).
My vet told me that kittens are not territorial, so I would not think it had anything to do with territory at that young age. I don't know how old Bella is, but take this under consideration. Also does Kitter need to be spayed/neutered? If s/he has strong smelling urine, it makes me think s/he is nearing that time.
Before spaying, my Bengal kitten had very strong smelling urine. After spaying, my Bengal kitten had normal smelling urine. What a huge difference! It must have been driving all the other cats crazy.
Good luck to you!