Whats wrong with my cat. Help please!?

We have 3 cats. One is a girl, the other two are guys. My cat, the girl is second youngest. And for some reason, and it just started today, she walks on her back and front legs, and slumps her belly down like somethings wrong with her back? Or her stomach? She usually bites and scratches, but she has been nice and cuddly lately. She doesn't have a bigger stomach or swollen , so I don't think she is pregnant. When she lays down she keeps scooting forward and has her in the air, and never lets her lay down. I kept petting her, and she rarely purrs, and she has been purring a lot, and for no reason at all lately. I don't know if something is wrong with her back, if she is "frisky", or what..also my littlest cat, a few months old, and my girl cat like playing together,but my oldest cat, a year old, and my girl cat don't like each other, he always try's to hump her, and she scratches him. She keeps rolling around also. I don't know I need help with this though.