I have to admit I'm not the best person to ask about allergies, and their effects on the whole system. There could be a connection, but I'm not sure.
By far I think the best solution would be to take her to the vet and take a urine sample, in case there is anything about her urine which could be causing skin inflammation on her vulva. If her urine is acidic, for example, this might happen.
She has been spayed. She was when she was 6 months old! I'm not sure why I didn't think about this earlier, but she is the same one that got stung by something out in my yard the other day. Could this have anything to do with it? I'm thinking that she was allergic to whatever stung her.
If she hasn't been spayed, this redness and swelling is most likely her "heat" season. Females get swollen around the vulva area in the middle of the Heat cycle, usually about 10 days or so after the start of the bleeding. This then can last from 3 days to 1 week.
If you think this might be it, it is quite natural and nothing to worry about.
However....your next worry will be how to keep all the boy dogs away!! Otherwise she will get pregnant. Because at this stage in her Heat cycle she is ready to mate. For those few days in the middle of her cycle, she will be offering herself to any male who comes near!
So that's if she hasn't been spayed.
If she HAS been spayed, then there could be some irritation or inflammation going on, which could be caused by a few things. The best option in that case is to take her to be checked by the vet, and take a urine sample with you.