Hi Suli - thanks for reaching out, very sorry for the delay in getting back to you. With your PrePaid service, if you've used more than 512MB in a day then another $2 will come out of your credit. If you've done an IOU the day before you've recharged, when you do top up the IOU will come out of your credit. It could also be premium sms. Would recommend logging in to your My Account here to check the usage history.