How does my facebook keep getting hacked?

[Gracie<3]

My facebook keeps getting hacked and I dont know how?? I mean today I changed the password because someone sent some of my messages to someone else {I didnt think anyone knew my password}. So then once my password was changed to something no one would guess, I logged out. Then about an hour ago I tried to log in and it said my password was changed AGAIN from a different computer, at a different time than when I changed it. So then I requested a new password again and no one knows it because I changed it when I was home alone...but I dont know how I keep getting hacked???? No one would ever guess my old password....and my new one they would never guess but they apparently did because I was hacked once again. Its bugging me so I just need help figuring out how they're doing this. Okay thanks.