My 5 pound chihuahua has milk coming out of her nipples. I haven't bred her. I'm worried that she got pregnant by a 12 pound dog without me knowing. What should I do?

Take her to the vet as soon as possible. Her and the puppies could die if she has them naturally.

If you think that there is a chance that the male dog bred her then the possibility is certainly there.
Depending on the time frame your vet could tell if she were pregnant by 1-actually feeling the puppies 2-by ultrasound and 3-by doing an X-ray. The fact is that given the size difference between her and the male if she is pregnant that delivery could be very difficult and dangerous for her. It would be best to know for sure before that point so that you can get her properly cared for during the delivery should she have problems. And after this scare is over you should consider having her spayed so that you don't have have this to deal with again.

How long ago was she possibly bred? Milk can appear anywhere from 3 weeks along to the day of birth. Determining pregnancy off milk production alone is not accurate.

It is possible for little dogs to have only one or two puppies and they will not show their pregnancy the way they should. And that is why everyone who as answered you so far are concerned about her birthing them. Big puppies passing through a small birth cannal don't end very well.

There is a chance that your dog is also going through a false pregnancy. These are not rare, however; I would suggest to get her spayed. Once a dog starts going through false pregnancies, they will go through false pregnancies after every heat cycle. Spaying your dog will also eliminate the possibility of her being bred to a bigger dog.

You need to call your vet. If your dog is producing milk, and you think she might be pregnant, then you need the expert advise from your vet. There is a good chance she is not pregnant, in which I would get her spayed ASAP.. and then again she could be pregnant.. if so, then you need to start preparing yourself and your dog for the pups.