If you have not had sex, you cannot be pregnant! The links listed above will be helpful for you to learn more about things like pregnancy and your period. Good luck
Btw, these are some good resources of sexual health information, specially for teens. Try to take a look: http://www.scarleteen.com
https://www.plannedparenthood.org http://stayteen.org https://sexetc.org
Hi there! You're young! Your body is still changing, and your period is not regular yet at 13 (well, It can be regular, but it's totally normal not to be). That's probably why you missed your period. You'd probably have felt something weird, as a virgin, after being sexually assaulted. but, if you're really worried, and have access to pregnancy tests, try taking one, so you can relax (:
You need to take a pregnancy test