Just takes my money

I got a personal loan for school fees. Which I then entered into a contract as I had the money to pay the school fees in a lump sum. School fees only due beginning of December. I decided to store the money till then in my credit card. Fnb then decides to decrease my credit card limit with the amount due for the school fees. Without notice or any form of contact. I am now R15000.00 short and in breach of contract. Considering I have never defaulted on a payment. And with my Christmas bonus always dump more money into my credit card to decrease limit. How can this be legal. How is this fair? I am now in very serious trouble.