I had a similar problem, but for over £1000. I have little doubt that I had used the electricity, but they had reduced the Direct Debit and it wasn't enough to cover the bill. They did say I could pay back monthly by increasing the Direct Debit payment so I did. They have just done the same thing again and I have a feeling that it is a good way of stopping me changing to another supplier (Cynical?)
Check your meter by switching everything off and see if it is still turning. If it is, ask your supplier to come and check the meter.
I would like to cut my bills, but with a DBH and teenage twin GDs living with me, it would seem impossible. So much washing, drying, an inability to turn lights off or any other gadget, I am at a bit of a loss.
Good luck Hilda