Dog adopting our new kitten?

We rescued a kitten about two weeks ago, we are not sure on it's age, but we are guessing it is now 7 weeks old. It showed no interest in milk replacer, but went at the kibble more than enthusiastically.

I have had my miniature dachshund for about 4 years now. She is not spayed, and has never had a litter. She has had no access to any mature males (we have one puppy who is too young to neuter, but plan on it once he drops).

Since yesterday, it appears my dog has adopted my kitten. Her nipples are swollen, and she is extremely protective of him. She has never been aggressive, but now I am the only one near the kitten. She lets him nurse, massages his belly, and eats his droppings.

Why is this? How long will it last? I had the kitten potty trained, but now he goes wherever he wants because 'mom' will clean it up.