They say the girl suffocated after lighting a fire to keep warm.
Under an ancient Hindu practice, called chhaupadi, women who have their periods or who have just given birth are seen as impure.
It was banned by the Nepalese government in 2005, but still continues in remote western rural areas.
The body of Roshani Tiruwa was found by her father last weekend in a stone and mud hut in the village of Gajra, in Achham district, 440km (275 miles) west of Kathmandu.
