100 bucks says...

What have I done this time? A while ago, my credit limit dropped by a mysteriously inconsequential 4 dollars. A few days later, it was raised by the same amount. I just checked my accounts again, and it seems my credit limit has been reduced by 100 dollars. It's still inconsequential. And still mysterious. Also mysterious is that I have a balance of $30.19, but no recent transactions. Why is it that they bother to tell me how much I am in debt to them without telling me how I incurred that debt?