I had a cat once that I had found as a stray, and she did that exact same thing. She would be purring and happy, and then 1 second later, bite us. I do not really have any good advice, other than maybe take her to the vet to see if something is bothering her. Maybe the fact that her previous owners did not handle her much, treat her right, and just let her go to fend for herself, that could be just her nature now. My cat never got over being dumped and mistreated... We just warned everyone that she has been known to bite, and tried not the handle her much, and at long intervals. Some cats get overexerted easily with alot of petting and things. I hope this helps, although I am sure others will have better advice Good luck... Katie