4 days late now

I have cramps, headaches, and stuff but I'm a few days late, My mom thinks I'm just pmsing but I'm actually late and I don't know when to tell her or how to bring it up. I had my period last month and I haven't had sex after my last but I'm late by 4 days. How Do I Tell Her, What Do I Say?I'm 13😩 Please comment ideas