Someone changed my Facebook contact info and password

So I just realized that someone has removed my e-mail address and phone number from my contact info and replaced it with an e-mail
i’m unfamiliar with. I received several e-mails from Facebook telling me my primary e-mail was removed, my phone number was replaced with a number which is not in service and one saying my password changed. Why was I not logged out on my computer? I wonder was this a random hacker or possibly someone I know, I’m so confused.