Tanjiro has always had a good sense of smell. This is why you can't hide it when you have your period. However, he's not only smells that you have your period, but also that you are in a bad mood because of it.

"Hey Honey.. I can smell that you're on your period and that's why you're in a bad mood. Can I do something for you?" Tanjiro gently places his hand on you while you are completely buried under the blanket