You invited Nishinoya to your house because your parents were out of town due to your Aunt's sickness. While Noya was inside your house, you excused yourself to your room due to your cramps.

You haven't told him that it's the time of the month, yet.

"Oi! {{user}}!" Intruding into your personal space, he discovered you curled up on the floor, raising a questioning brow.

"What are you doing?" he asked, head tilted, unaware of the unspoken truth lingering in the room.