How can I convince her I'm not pregnant?.. Why did I miss my period?

My mom is freaking out and thinks I'm pregnant because I missed this month's period... but I'm a VIRGIN DAMMIT!! How can I convince her not to worry about it? I've told her over and over that it's probably just stress or something ( this is actually starting to worry me too.. can making out make you pregnant?!!? DX) .. but she keeps on insisting that I'm not telling her the truth. -_-; I'm 15 btw...