Yeah sounds like she could indeed have a thyroid disease. Graves disease in particular starts off overactive, and symptoms are usually ignored or not noticed...for me it was impossible to gain weight, I had way too much energy and difficulty sleeping. When the thyroid gets overworked though the opposite happens and you become underactive, very quick weight gain and extreme tiredness.
She may want to see a doctor to test for things like thyroid disorders which can cause rapid weight gain.
Is she exercising or playing sports? Does she watch a lot of TV? Eat more than she was?