It means she is going through the hormonal changes of an adult female.
The nipples pop out when a dog comes into heat for the first time.
If your dog was recently in heat and could have been bred by a male then its very likely she is pregnant.
This site goes over signs your dog is pregnant http://www.debbiejensen.com/signs_of_pregnant.html
If you know for sure she was never with a male this could be a false pregnancy though which you can read about here
You can read more about pregnancy and whelping in dogs here
You may see a sticky clear discharge around week five or so indicating pups are present in the uterus.
Hope this helps you!
Sometimes a male dog will check back to see if she is still in heat.
If she was outside and unsupervised and there is an intact male in the area she could very well be pregnant. Male dogs are very clever at finding females by their scent and figuring out how to get to them so they can mate.
If she has a sticky discharge that is one sign. Do read the website on how to tell if your dog is pregnant.
Your vet may also be able to help you figure out if she is pregnant or not.
If she is yes it would very much be best for her to have the pups inside if you want the pups to live.
I prefer all dogs as indoor dogs as I like to have them with me and most of the year the outside is nowhere near as nice a place to be as inside is.
There might not be any stray dogs but some neighbors male may have gone missing for an hour and that would be enough!
Well when I say neighbors - my little 15 pound dog, when I was a kid many years ago went 8 miles away to visit a neighbor female dog he scented!
This pregnancy will not impact a later one as the sperm that meets up with her eggs will be different.
Do read the website above about the signs and pregnancy in dogs.
If she is big, if you feel pups moving, if she is also in milk she could be getting ready to have pups.
Normal pregnancy lasts 58-65 days in dogs counting from a first breeding which usually takes place about in the middle of a heat cycle. So if you know when she was in heat that will give you an idea of when she might be due.