why do some girls do this?

Why do some girls just make up lies about you and spread them around? Like how can you speak to people knowing you're lying to their face? I don't get it. 
Yet, people continue to believe them and nobody will ever know they are a bold faced liar who just makes things up off the top of their head. Multiple girls now. 
The biggest lie being that my mom caught me having sex in her bed. 
IF THAT HAPPENED I WOULDNT BE ALIVE RIGHT NOW. That spread around school like wild fire. 
Also
Apparently I slept with someone's boyfriend. This girl, I DONT EVEN KNOW WHAT HER BOYFRIEND LOOKS LIKE. WHO IS THIS DUDE THAT I DONT KNOW THAT I ALSO FUCKED? CAUSE I DONT KNOW. PLS TELL ME. WHEN. HOW. I DONT LEAVE MY HOUSE. DID WE DO IT AT SCHOOL?