A female dog's vulva will swell during their heat cycle. Heat cycles typically last for 12 to 21 days depending on the individual dog.
Does A Female Pitbull Vagina Swells When On Their Cycle?
Yes, very much so. Sometimes 4 times their original size when not in season.
I think it is the same with all female dogs. I have got 2 labrador's and theirs swell up when they are on their cycle