Unknown Charges To My Account

My name is Lyn Nicholls and my husband Stephen and I had cabin 1046 for one wk starting 13 May, Booking No 138MD8, voyage CO20120513007, I am looking at my on board account that was paid in full including gratuities, the lady who took my money assured me no other amounts would go onto my Barclaycard, I have just received a Barclaycard bill with £28.55 put on by Carnival Conquest, I hold al receipts which match my bill, please explain what this is and notify me that this amount had been paid back into my account. You are very hard to get hold of once back inthe UK.