Since the vulva is swollen I would suspect that she has recently been in heat or may just be finishing her cycle. The only way to know for sure if she is pregnant at this point is to have her checked by a vet. You can have her spayed if it is early in the pregnancy. It is easier and safer to do if she is only a few weeks into the pregnancy. You need to find out now. If you wait it is more expensive and much more dangerous to the female..... Or you could have puppies on your hands. Small dogs have more difficulty delivering puppies than bigger dogs do so make sure you are prepared for possible complications if you choose to let her have the pups.
I hope you can help this poor little girl whatever the case may be.
