Period and brothers? PLZ help ?

When I'm on my period and it really hurts my mum let's me stay off school for a day. But my brother will say I'm sciving and be mean to me every time I say something to him. He will call me sciver which makes me feel awful I've only ever scived if I CBA to go to school which is rare. I have told him that I get bad cramps but he doesn't understand? I'm going in to school late because of him I don't want him to be mean to me....any good ideas?