Probably hacked by someone ,
First of all , Logout/Delete your account from devices , i.e Your phone , PC ,etc
2)Change password from your PC ( Probably Google account )
3)If you don't watch youtube from phone , Don't Login in Google account on your phone
4)After changing the password Login again from your PC
If still someone is hacking you may need to ask Google or make a thread at their forums
Good Luck