First of all, I would confirm the Neg- reading with her doctor via a blood test as this is much more accurate than any home test. Do keep in mind though, there are several other causes for an irregular in menstruation as well. Examples; stress, weight gain/loss, medication, illness, cyst, or polyp of the cervix, hormonal imbalance, thyroid, infection.. just to name a few.
It is possible with her history of yeast infections, that is can throw off her cycle, but still confirm the Neg- readings with a blood test.
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