Hello, I'm Dr. Deb and will do my best to help you today.
It's certainly possible that Baby may be pregnant if she mated with a male dog. The gestation length for females is about 63-64 days just to give you an idea of the time line involved. However, I wouldn't expect her nipples to be enlarging so soon if your timing is accurate. She would only be about 4 weeks now and we typically don't see signs of mammary or abdominal enlargement until the females are around 6 weeks.
But, it's equally possible that she's going through a false pregnancy which is basically exactly what it sounds like: a female dog will have all the signs of pregnancy (even milk in her glands) and yet there won't be any puppies to deliver.
The best method of identifying whether she is pregnant or not at this stage would be an ultrasound. If she is pregnant, then I typically advise that an x-ray be taken about 10 days or so prior to delivery so that an estimate of the numbers involved can be made.
A vaginal discharge can sometimes be seen during pregnancy but much depends on the color and amount as whether or not it's something we need to worry about. Of course, dogs about to give birth will develop a discharge as well.
I'm a little confused by your timing, though. I wouldn't expect her to be getting close to delivery for another few weeks at the earliest.
Given her mammary development and abdominal enlargement, it might be prudent to have her checked by your vet so that a clearer idea can be determined as to her pregnancy status.
I hope this helps. Deb