This might be an infection called a stump pyometra where the remains of the uterus is getting infections. Particularly since it seems like she might be having the infection in a cyclic manner it could be a bit of ovary was left in there too. Might be worth ultrasounding if she gets another infection to see if there is a stump area that is inflamed.
It could be a vaginitis and you might find swapping foods to one with fewer or no grains and higher quality fats and protein sources could help with that.
Innova Evo is a grain free food. California Natural Lamb only has rice which most dogs tolerate well.
Sometimes the high grain foods encourage yeast growth as the starches break down into sugars.
Hope this helps you!
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