Your monthly Direct Debit to your energy supplier is just a ‘estimate’ cost that’s suggested by your supplier to cover the monthly energy you’ve used - it’s never completely accurate.
At the end of winter, you might be in debit, which is the exact opposite of credit. Debit is incurred when your monthly Direct Debits aren’t enough to cover your usage. This is completely normal, and irons itself out when the warmer months come and you start using less energy.