Well, it'd be nice to believe the telepathy bit. But if you've had no history of it before etc, the chances are it was #3.

Maybe you'd just had a bad day, or you weren't feeling too well. It happens to everyone. I'm usually really happy, and bouncy, and whatever, but occasionally I get really depressed for no reason. Same with you?

EDIT: Just realised I missed Draco's post. If someone in a group is really really happy, then other's in the group will start to share the feeling. If one is extremely depressed, other's might feel their moods dampened. If someone yanws, everyone yawns. Something to do with human nature.

Though how would the emotion be transmitted over a large distance? Twins are said to be able to do it, but what about everyone else? We could've had a whole neighbourhood in Sweden jumping off bridges.