My 7 1/2 month old long-haired chihuahua will occasionally poop in the house. She is taken outside a minimum of 4 times a day and usually will pee and poop in the morning and when I get home from work, however sometimes in the evening after a long walk and a stop outside she will come in and poop inside. Is there something I am not doing? I have cats as well, however their litter is in a room the dog can't get to and she doesn't poop near it. I have never had a dog before and am unsure of what to do. She is really well behaved. She is locked in my kitchen when I am at work and sleeps in her crate at night with little complaint. We have started basic training and she has sit, lay down and stay pretty well. Does this mean I have established the dominant spot in the house or could she be pooping as a challenge? Thanks!

She could be pooping as a challenge like you said bit because she is so young it is most likely she is still a little confused about the whole 'toilet traiing' thing.

You need to be very, very vigilant about toilet training, if you see her poop say 'no' and take her straight outside.

You can usually tell when a puppy is going to poop as they start sniffing the ground and do little circles, if you see her do this grab her and take her outside.
You say she mostly does it of an evening? Then make sure you watch her extra hard at this time to cath her.

Finally, buy an enzyme cleaner to remove any odor (even ones you can't smell) from the carpet to make her forget about he places she has already pooped.

Make sure the give lots of praise when she does go outside so she knows it is a very good thing.