I have heard that cats purr when they are happy and they also purr when they are in a great deal of pain. Nobody knows why. some of my cats purr very quickly and some hardly purr at all. I guess they are all different.
It is not a stupid question. No question about an animal on PT is stupid and we all gladly answer as best we can. I'm sure if you have read some of the posts we come up with then you will realize that most everything has been asked here!!