My cat wants to sit in the dark bedroom?

I think it's to get away from my dog he's a chihuahua and he tries to Juno the poor cat all the time. But my cat who's a make gas a heart condition and it worries me^ he's spending so much time in his own in the bedroom. He hardly eats when he's in there I just want to know if he's depressed or something? At night when the dig is in his cage the cat will come and rub his head against me^ sit on me^ and purr. Is he lazy? Or could he be I'll?