Period and Eating

Honestly, when it’s around the time of my period or when I’m on it - i don’t have an appetite at all. My dad thinks I’m becoming anorexic (i just can home from College so we hasn’t seen me) and he’s complaining that i don’t eat much as i used to and continues to ask me what i ate today and pressures me to eat at dinner when i simply just don’t have the stomach to eat. How do I explain this to him?