WordPress hacked? or just bad plugin

Hi,
yesterday I went to bad happy and everything was working just fine. Although I installed many plugins and tweaked some of them, no problem was apparent. Then I wake up today and couldn’t log in in any way to wordpress admin page. I tried to recover the password and the user name was changed to ws for the main account and 3ca-61 for all others. I got to phpmyadmin and changed them and it worked, but now i’m worried about security. I’m not sure a plugin could have done it, but I guess it is a possibility. Have it ever happened to you?