Trouble with Multiplayer

I only just got Gmod for myself and a friend via the steam store, single player seems to work fine, but whenever I try to play multiplayer it gives me a pop-up saying my account is used elsewhere, the only option from there is a button w/ login on it, but that seems to have no effect.

I was able to play Tf2 just fine, so I’m not sure if it’s a steam problem or what?

Any suggestions? This is just bugging the crap outta me.

Change your password and don’t tell it to anyone next time.