I believe this was a false alarm, something topical rather than illness.
I gave her a long, cool bath yesterday, using a shampoo that "relieves itching associated with minor skin irritations, inflammation and rashes."
She has completely stopped licking (anywhere on her body, not just under her tail). She seems much more relaxed and comfortable, compared to yesterday, even though we're dealing with 99-degree weather.
When I check her labia, I don't see any redness, or any change. I can prod the area, and she doesn't show any sign of pain.
I think that what I thought was swelling is just how the vulva looks in a healthy dog. I'll keep a watchful eye on her for the next few days to see if anything changes.
Thanks for the tips. 🙂
(Waving hi to Laura and Ziggy's kin)