Somebody is using my Youtube Account

I noticed that somebody is using my youtube account, it's been happening for a while, couple of months, i notice videos are "liked" and added to favourites which i never added myself, also today i went "history" and apart from the videos that i actually watched theres a lot of videos of some singer that i never saw beforve.. I tried changing my password for my gmail account, cuz that's how i sign in to youtube, but it didn't work.. does it mean that this person knows my gmail password? But after i changed it logged in and out a bunch of times how could he still log in? What should i do?

Instead of asking us.. You should ask admin or the help section at Youtube. I'd think that that would be the first thing that you would think of. *trys to ignore the hair colour*

It must be your ex .