How old is your dog? Is she spayed?
How long has she been swollen? Is she licking there?
Any changes in urinating? Does she scoot/ drag her hind end on the ground at all?
She's in heat. As they get older, their vulva swells more and more with each heat. Some dogs have bloody discharge for the 3 weeks they are in heat. Some have what we call a silent heat with no discharge apparent. You can check for discharge by trying to stop her licking for 10 min then wiping a white tissue down the center of her vulva, top to bottom. Between the folds. You may see discharge then.
The swelling this badly should last at least 3 weeks, but may not return completely to normal as she ages. It will swell more with each heat, which can occur every 6 months to 1 year in most dogs.
Let me know if I can help further.