I'm no professional, but it sounds to me like she's giving you a "love-nip"...My cats do the same thing, so I don't worry about it...
My Cat "Bites" My Arm When She Is Purring And Kneading. Doesn't Hurt, Doesn't Suckle Or Lick, Just -very- Firmly Holds Some Flesh In Her Teeth. What's Going On?
My cat does this sometimes, too. Try and see what you're doing before she bites. It may be that you're petting her in a way that hurts her or that she just doesn't like. My cat only likes it when I scratch behind her ears, and will nip if I stroke her back.
My male cat does this too, but not to me. It seems that he knows it would hurt me. He does this when he is kneading a blanket, pillow, or a spot on the couch. He will bite the blanket firmly as he kneads and just hold it in his mouth.
My cats always do that to me. Their probably just playing or are in discomfort.
So does mine she seems happy but wants my full attention or will bite me! Just pay attention to her while petting.
My 1 year old male domestic does the same on not to me. I've only had a year of Vet school so I'm not an expert but I can give you advice. See, when cats knead they are marking their territory as well as displaying whoever they are near love. When a mother is nursing her kittens, the kittens will knead the mother for stimulation of the milk. It's natural for cats to knead. The "biting" on the other hand I have different theories. See, my cat will move the blankets with an end in his mouth as he kneads across my bed. Every now and then he will get a different area of the blanket to move. But when he finally lays down he chooses a spot not affected by the moved blankets. I have believed that he is creating comfort for me as well as biting down to be sure he doesn't injure himself while kneading.