Major dog problems, I'm starting to worry, help?

Hi, so it is about my lahasa apso, Pixie, about 5 yrs. old, for two days, she has not once pooped,only eaten a few nibbles from her food ( I mixed in some dog food, since I'm trying to convert her diet from human to dog food), never wants to walk when we're there or with a leash ( even with food or toys), she does not want to catch the ball, SHE IS NO LONGER THE DOG I KNOW! I am so worried, she is only normal when there's a cat around... She just sits there, how can I train her if she's already sitting! I'm both worried and annoyed. She has been in a cage for months since they were having severe cases of ticks! and no, we haven't moved to a new environment, we're still living in the same environment, when I bought her. Please, I am begging, help!