Cats purr for many different reasons, often because they are content and happy. Sometimes though the purring is a sign of anxiety, illness or stress, especially if accopmplanied by drooling or a body posture of ears flattened. Scientists still don't know what part of the brain controls purring but the sound is made by air passing through the vocal chords at a frequency of about 25Hz .
Researcher Elizabeth von Muggenthaler of the Fauna Communications Research Institute in North Carolina (FCRI) is a specialist in the field of bioacoustics. This is the study of the frequency, pitch, loudness, and duration of animal sounds related to their behavior. She was one of the first to pull several strands of research together, and propose that felines do gain evolutionary healing advantages from the purr.
Many experts theorize that the 25 Hz frequency of the cat’s purr might offer a kind of built-in physical therapy. It’s probably no accident that this frequency is also used in humans to help wounds heal faster,
So your cat may be content , attention seeeking or trying to make itself "better" !