Central air can use a lot of energy, so if you are running it more that can make a big difference. I lived in the Austin area and the rate for electrical use changed once you passed a certain amount per month. I think it was 500 kW. It wasn't a small change either. It went from 3.55 cents to 7.82 cents. It could be that you have the same situation and the extra energy you are using is pushing your usage up into the higher rate. Your bill can grow pretty quickly when this happens. Also, the high rate in summer was higher than the high rate in winter. I was always surprised by the large variation in my bills.
So just as an example, for a 500kw bill you would be charged $17.75. For a 1000kW bill you would be charged $17.75 for the first 500kW and $39.10 for the second 500kW for a total of $56.85. Double usage but almost three times the bill.
It can be hard to use less energy. One simple thing you can check on is if your water heater is insulated. Doesn't make a huge difference but it can help. Also, check to see if you get charged for peak-hour usage. If so, you can wait to do things like washing/drying clothes and running the dishwasher until off-peak hours. Switch to CFL light bulbs. Some cities and/or utility companies offer assistance paying for energy improvements such as caulking and weatherstripping. That may be worth looking into.
Austin used to have a program where they would take your yearly usage and average it out by month. I don't know that you ended up paying less over all, but it did give a fixed bill per month which makes budgeting easier. Perhaps your utility has a similar program.