Ah, cats. The furry felines are as mysterious as they are beautiful, and cat lovers adore them for it. From the way a cat moves her tail to the specific positioning of her ears, determining precisely what she is trying to communicate is not always the easiest task!
If you've ever noticed that your adorable kitty seemingly automatically sticks her rear end up in the air when you pet her, it's because she loves what you're doing and wants to feel it more! The area by a cat's tail is an extremely stimulating and sensitive spot, so by sticking her rear in the air, she essentially is trying to maximize the "feel good" factor. It's actually a compliment. Your cat is enjoying the touch and wants you to continue doing it. This is pretty straightforward, really.
The word may sound funny and awkward, but "lordosis" actually is a technical term that is used to describe a posture female cats assume when they are in heat. Sexually mature queen cats stick their rear ends up in the air to indicate to males that they are prepared and ready for mating. Cheeky little things!
Tomcats frequently exhibit the stance when engaging in typical territorial marking behavior -- the infamous urine spraying. The position has no deep meaning when done in this situation, generally. Male cats likely believe that their spray will hit their intended target more accurately. Some cats also seem to think that they will look more imposing and threatening with their rear ends positioned higher.
If your female cat is positioning her body this way constantly when you're not even petting her, consider taking her out of her misery and getting her spayed immediately! Once she is spayed, she will no longer go through the heat cycle, thus saving you a lot of hassle. Not to mention, it eliminates the troublesome risk of her becoming pregnant and contributing to the ever-growing epidemic of feline overpopulation. Cats that are spayed also enjoy the benefits of improved health, and honestly, what could sound better than that?