Barclays Credit Card Complaint

Barclays Credit card department,

APR or interest rate New Jersey

I have a - credit card through Barclays and I was just charged some kind of deferred interest, of which they provided no advance warning, of over eight hundred dollars in a single charge. I was steadily paying off this balance and was - - dollars below my credit limit, and now I am, out of nowhere, - - dollars over my credit line. I do not understand why they are just allowed to do this, which completely messes up my credit and short term budgeting, which no warning or clarification. Please help me deal with them to try to get this egregious charge taken off of my account. Do n't they have to provide notice or explanation? If they were about to charge me - - dollars of interest, should n't they have notified me, since they notify me automatically every - month when a payment is due.