Citibank Credit Card Complaint

Citibank Credit card department,

Overlimit fee California

I have a Macys credit card and i wanted to place an order which never in the past i had an issue and when i was on the chat screen with someone from Macys they told me i was past my credit limit and i asked how much and they well your limit is $600.00 and you are at $800.00 and something cant quite remember the exact amount. but i said " WHAT! '' how can i be past my limit when i was never told i was past my limit and to this date i get emails from you saying to pay the minimum due which is $29.00 and i pay $30.00 or more! How can they do this isnt this a ripoff?? if im at my limit there should not be any more charges past that amount!! I should only be charged the $600.00 which is my credit limit! please advise me or let me know what to do