I mean, she'd be mid-30s to mid-40s, and in her 30s when this was written (2018).
But how does that mean she had menopause in her by age 37???

Edit:
Did you misread "first period" as "period", and think she meant she thought that menopause was the dog impregnating her???
She meant thought that she constantly bled, not a few days a month. It's a double whammy for the Ontario school system: what to expect from a period, and how impregnation works.