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Is she spayed or intact?
Could you take a picture of the swelling and post it here as an attachment?
It looks like she has a bit of a secondary staph infection or yeast infection of the vulva. That skin looks red and irritated. If she's licking at it, we need to get an e-collar (cone collar) on her to stop that.
To help the infection go away, you could either use some topical anti-biotic wipes or just start her on oral antibiotics from your vet. There is a product called Malaseb wipes that are available over the counter and online. They have some antibacterial and antifungal effects.
If you notice any purulent (pus) discharge from the vulva itself, then I would get her seen right away. She is definitely at risk of pyometra- an infection of the uterus- from ascending bacteria up the vaginal vault. If that happens, then emergency surgery to spay her is needed. Hopefully that is not going to be the case here.
I hope this helps, let me know if you have any other questions.
That should help a lot within 2-3 days.