Signed in fr Diff PC ?!

Thanks very much for all your ongoing help.
Today when I clicked on an email Link in an email i received,a Message Box appeared:
"You have been disconnected from Chat because you have signed into Yahoo!Messenger from another Computer or device. "
I did not click on the OK cuz I did not sign into anything from any other PC or device.
I closed the box. But it keeps reappearing when I click on that Link.
(I have clicked on that Link many times before and this never happened.)

So then does this mean someone is using my email add to sign into Yahoo!Messenger Chat ?
If so, how can i prevent/stop this ?
Thanks very much for any info you may have on this.
from marj