Why did i have this bad dream, i didnt even THINK about?

im 12, and i had a dream that i had sex with my bro.. I didnt even think of any of that!! before i went 2 sleep. But when i woke up i started crying, cause my dream was gross and eww. HOW DID I HAVE THAT DREAM??? am i just a DIRTY-MINDED PERSON OR SOMETHING??? i just cant get that dream out of my head.