Some Hindus view menstruating women as impure and in parts of Nepal they are forced to remain in a hut or cowshed for days, a practice known as chhaupadi.
"We are investigating the case. We suspect that she died of suffocation from the smoke of a fire she lit to keep herself warm," local district inspector Badri Prasad Dhakal told AFP.
Under the practice, women are banned from taking part in normal family activities during menstruation and after childbirth, and can have no contact with men of the household.
The government outlawed chhaupadi in 2005, but Mohna Ansari of Nepal's National Human Rights Commission said local leaders must do more to enforce the ban.
"We have a legal ban but the law enforcement forces have not been strong about implementing it," she said.
"It is crucial for us to work to change the attitudes of the people and raise awareness against this practice."
