Well you were either on a very high tarriff and they changed it, meaning you've been overpaying for your fuel; or you have a lot of credit meaning you were overpaying on your direct debit and they haven't refunded you, emmie.
You really should know how much debit/credit you are in at any time.
Yes I do find it difficult to understand how people can pay out their hard-earned to a company and have little or no idea how much they should be paying, what they have received for their payments, whether the company owes them any money (or vice-versa). What makes energy companies so different in this respect from any others? It is not difficult to work out roughly what you should be paying under any particular tariff and whilst you may be a few quid adrift but you should not be out by a factor of two or three.
I'm with E.on and pay, at the moment, £41 a month for gas and electricity This will reduce when the summer comes and I don't use the central heating. I'm on a fixed plan for 12 month, live alone in a small bungalow.