I'm guessing you're American because apparently 16 isn't age of consent but maybe she's just giggly, maybe you make her laugh, did you make a joke do people nomally smile around you, my sister the same but that's cause I remind her of her younger slightly autistic brother (same form of autism as me but I've trained myself to hide it beneath general quirkiness) and she then smiles and laughs at memories of his antics, which I have to say from what I've heard are actually comedy gold so maybe not flirty.
But the again I'm an unattractive social awkward, prudish and innocent bisexual guy with low self confidence so I don't really know what to like to be flirted at or crushed (or have I an not noticed) on or even in a relationship so I'm not credible
Sorry for long reply I don't really do small talk I get on to the meat of conversation