Q: What does it mean when a kitten keeps her butt up in the air?
A: You don’t give much detail in your question. However, when I hear that a kitten is putting her “butt up in the air,” my first thought is that the kitten is in heat.
Heat is the phase of the reproductive cycle when a female cat will allow a male cat to mate with her. Most cats come into heat around 6 months of age, although it can occur several weeks earlier in some cats. Female cats will actively seek to get bred and will exhibit behavior such as increased vocalization, raising their rear ends in the air, pushing their tail to the side, and treading their feet in the carpet. You can induce or exaggerate this behavior when the cat is in heat often by simply touching the cat near its rear end.
You should confirm my diagnosis with your veterinarian, and if your cat is indeed in heat, she should be spayed promptly.