"You have been disconnected from chat because you have signed on to another computer or device?

Sometimes when I get on my computer, that message shows up on Yahoo. I'm not sure what it means-is someone "piggybacking" on me (I don't chat or Im or anything)? The only other person in the house is my boyfriend, and I know for a fact it's not him either, he hates social networking. And in case you haven't figured it out from both clues, yes, we are both a little old.