Empty sex can mean a couple things,
She has no feelings for the person or it was just physical enjoyment.
Or she has as we call in the PUA community "Buyers Remorse"
More of less at the time she had attraction and was getting swept up in the moment, but the next day she regrets it, just lke when u buy something ur like ahh i should return it :P buyers remorse.
Basically need to form more comfort with her and try and spark a connection.
Sex is very mental for most women, Capitalize on that especially while in the bedroom