Kurasawa Mota manga
From Fateful Encounters: Story1 - Junior high third year Yue is convinced that being a girl really, really sucks because of... that you-know-what time of the month. But her opinion starts to change when she stumbles into the infirmary one day and accidentally comes across a certain boy named Sagami!
On a cold day, Mirita discovers a hidden room with a kotatsu (a table that keeps one warm) inside. The kotatsu belongs to two of the most popular and handsome twin brothers in school. Both immediately kick Mirita out. However, Mirita becomes even more determined to get back into that room. What love triangle will commence?