I can't actually tell if this is because she gets confused and forgets they are hers, or if she gets so excited that even though she DOES remember they are hers, she doesn't care. Third possibility - we could have a masochist kitty on our hands, to go with our cat that likes being spanked.*
*For those of you who have not met Pretzel, please do not be alarmed. He's just a really odd cat, and if you pat his butt, he sticks it way up in the air and purrs madly.