Itunes billing issue

I received a text message from the bank that I purchased something from itunes when I actually didn’t buy anything and when I tried to change my billing method, it was rejected. And text messages about purchasing something from itunes kept coming and deducted my account balance into half. I noticed that the time written on the text message where it says I bought something, is the same time where I kept changing my billing method. I checked the transaction history of my account and there was no record of a transaction. What should I do?