Has anyone ever had or seen a cat who sucked its leg?

I’ve read in several places on the internet that a cat can suck for two reasons. Either it was weaned too early, or it’s anxious. My Bo (the cat in the avatar and the “poof” in my name) loves climbing onto me somehow, and sucking her leg (an inch and a half or so above the paw on the right front leg) while kneading hard and purring like a buzzsaw. It’s definitely got nothing to do with anxiety because she’s so happy she can’t contain herself. I can’t see how it has anything to do with being weaned too early, as there is a criteria before she’ll do this: She must be on top of me, I must pet her semi-vigorously, and she must be touching me with one of the paws she’s kneading with.

Any ideas?