Sorry to hear that @Annmaree Finch. Have you recently changed your plan or changed your direct debit date recently?
Have you also used any out of contract charges at all? Such as roaming abroad, international calls/texts, MMS messages (photo’s/emoji messages), or used any out of contract calls, texts or data?
Furthermore, as advised by @Tom, have you logged into the iD Mobile app/website and viewed your bill, as it should show the reasoning for the increased bill this month?