You may be looking for the word "delinquent." Your account is delinquent. You are ten days past due on your payment.
In the strictest sense, as soon as the debtor violates the agreement (which includes paying on time) you can say he is in default. Often, however, you reserve "default" to be a very serious delinquency and the account will be closed or other significant action is pending.
In the U.S., government student loan terms are a little different.