It might swell because of natural causes.
What Causes A Dogs Tits To Swell After She Has Had Puppies?
It could be the effects of having the puppies feed or the milk drying up. I'm sure the area is very sensitive, so it is best to leave her be.
If she has stopped feeding then it could be her milk drying up so it may be a painful time for her if it doesn't subside get veterinary advice asap
From breathing in and out.
Best get it checked as it could be mastitis which is an infection in the milk glands. If she seems to become unwell best to get her checked out.