Best answer by Tim_OVO
I'm going to step in here, as it's unlikely that another user can advise on such an account specific query.
It sounds like your account has been billed up to date, which has updated your account balance. It might be that we had billed you to an estimated reading that was too low, and this updated when an actual reading was provided. Or maybe you only get a bill every 3 months. So your account balance was £0, and then changed after a bill was generated. This is all guesswork though. So that you're sure what's happened, reach out to the Care team via Facebook, Twitter or email: firstname.lastname@example.org.
First have a look at the actual statement in My OVO: http://ovoener.gy/myovo. The answer will likely be revealed there!