Nothing to worry about. I host the various sites on the same server, and normally, I only use about 1/10 of the daily transfer bandwidth that's allowed. For most of my folders, I have hotlinking disabled, so that no outside links can be made to the images on any of these websites. I had missed doing this for one of my folders, and someone had posted a link on a popular site, which resulted in a sudden influx of 15,000 hits to the picture, which quickly used up the remaining bandwidth.
Fortunately, the site's bandwidth is limited on a daily basis. So, if that should happen again, it will be back up as soon as the new server day starts and resets the daily usage back down to 0. I believe I've taken care of the problem, so hopefully it won't be happening again any time soon.