I'm sort of confused how that's even possible unless rates change. I mean, usage is usage .. no? Although with that said, I find the utility companies always find a way to get you at first. For example I just signed up for a lower rate and to compensate for that they billed me at almost twice the normal billing period so I still had to fork over a sizable chunk of cash. That said the next bill should be pretty nice! Hope so for you too. I just don't see how that they could even try to get away with such an increase unless they claim your usage has gone up (or your rates).
Good luck!