My mood. I cannot hide my emotions, at least not entirely. It leads me to stay true to my honesty- sometimes brutally honest, but that's been tempered with age. I'm very empathic and can tell how the crowd I'm in is feeling.
And thusly- whether or not I like someone or am simply tolerating them can be extremely apparent (often moreso to an outside observer).
So if you get this itchy feeling you really should leave, make your excuses and go- and if, conversely, you get the feeling I might be flirting, I'm trying to flirt discreetly. And if you're really unsure- it probably means that I, too, am unsure. If I'm unsure, then go ahead and be my friend, it'll do both of us more good than harm.