600 Dollar Utility Bill?!?

I live in Florida and we just got a 600 dollar utility bill, over a 400 dollar increase from our last bill. Nobody is at the house all day and we never leave the heat on when we're not at home. In fact, days go by when we don't even use the heat because sometimes we just don't need it. Last month we used 915kw hours, and this month we used "3,456"kw hours. I called the utility office and they apparently installed a new meter and someone is coming out to check it. In the event the meter is correct, what would cause such an increase in our bill?