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Out of curiosity when was her last heat prior to her being spayed?
Is there any discharge from the vagina?
Has she been licking the vagina excessively and scooting?
Is she urinating normally or making frequent trips?
Is she drinking large amounts of water?
Is she scooting so much that she is causing abrasions on her skin?
Prior to her spay had she had consistent heats?
We could be looking at a number of issues. If she is scooting then she is likely rubbing her vulva on the ground as well. This can lead to irritation and swelling. I am suspicious of this because all the surrounding area is red and swollen as well. I also suspect that she may have flea allergy dermatitis as a result of her fleas. Make sure we get that treated as soon as possible to get the fleas off of her. All it takes is one bite in a dog who is allergic to them to react as severely as it seems your girl has. Try giving her benedryl at a dose of 1mg per pound twice daily to see if this helps to control the itching, redness, and scooting. I also recommend giving her a bath in aloe and oatmeal shampoo to help soothe her skin.
The scooting could also be associated with her anal glands being full yet again. You can learn to express them here:
Otherwise I would have her visit a groomer to have them expressed for her. Adding canned pumpkin (not the pie filling) to her food will help to increase the fiber contact thereby increasing the bulk of the stools.
Now, lastly and this is what you do not want. She could very well be coming into heat again. This can happen with the vet performing the surgery does not remove all of the ovaries and leaves an ovarian remnant. This would require a blood test to determine if her progesterone levels are elevated. A swab test of the vagina can also be performed. I would consider taking her into the vet on Monday to be evaluated if flea treatment, benedryl, bathing and anal gland expression do not improve her condition.
In rare circumstances the anal glands can rupture from the inside causing infection that can travel underneath the skin and down one of the rear legs. This would be the only reason I feel there could be a connection. It also may be that the skin is so sore she doesn't want to walk on the leg. Other than that the leg issue may be unrelated to the vulva or anal glands. If she is acting ill or the swelling increases a trip to the vet would not be a bad idea.
Then I would be suspicious of a urinary tract infection which can cause irritation to the vulva especially if she has tiny tiny crystals in her urine because these can stick the inside of her membrane and cause irritation. Stones can also be a potential issue. I would seriously consider a urinalysis to rule this out.