In Jainism, menstruation is traditionally regarded as a period of increased bodily impurity, and women are often advised to refrain from certain religious activities during this time, such as entering temples, touching idols, or performing rituals. This is based on traditional interpretations and cultural practices, though perspectives may vary among individuals and sects.

If someone gets their periods while on Shikharji (Parshvanath Hill):

• Doāh (दोह): The term "doah" generally refers to atonement or expiation for an unintended transgression. If a woman realizes she has menstruated while on the hill, she should quietly withdraw from further puja or rituals and leave the sacred area as soon as possible.
• Prayaschitta (Atonement): After returning, she may perform simple atonement (prayaschitta), such as reciting the Logassa Sutra, Kshamapana (asking forgiveness), or performing extra samayik (meditation/repentance), according to her sect’s guidance.
• Intention Matters: In Jainism, intention (bhav) is very important. If it was unintentional and realized only later, sincere repentance and a resolve to be more careful next time is considered sufficient.

Ultimately, practices may vary by sect (Svetambara, Digambara, etc.), and it is always recommended to consult your community elders or your guru for guidance consistent with your tradition. The core principle is to maintain humility, seek forgiveness, and uphold ahimsa and purity in thought and action.