Translation:Is it hot in here, or is that just you?
This seems rather awkward. To me, "Is it hot in here? Or is it just you?" almost sounds more like a sarcastic response to "you" asking me "Is it hot in here?". I may be misinterpreting the original meaning, but I would think a more natural translation in keeping with flirting would be "Is it hot here, or is it because you are here?"
Wasn't too hard for me, but that's because I took Italian in college.
I understood using fare (fa, third person singular) instead of essere because of talking about the weather where we use it all the time. Fa freddo = it's cold. Fa piova = it's raining.
From there is was mostly just slightly uncommon arrangement of basic stuff.
qui = here, so "Fa caldo qui?" = "Is it hot [in] here?" "Ci sei tu" = "you are here", add the perché, interpret è as "is it" and you've got, "Is it hot in here, or is it because you're here?"