Daisy,
Mine went from $370 last month to $420 this month, but of course we have almost a thousand sq ft more than you.
I don't know. Have you called GCEC and asked them if they have any hints. I would maybe call an electrician too (like the one in your service manual, you all haven't been there two years yet, you are still under warranty!).
You haven't added any old appliances, right. Like an old garage fridge or anything... I am just racking my brain over here for you.