How can I tell if my yahoo account has been hacked?

I keep changing my password, typing the new password no more than 30 seconds after changing in and I am prompted with the message "Invalid ID or password. Please try again using your full Yahoo! ID, and type the text you see in the picture below." Does this mean that someone has hacked my account and is presenting me with the invalid password screen to prevent me from accessing my account? What should I do?