[Dark-and-Twisty]

Don't worry about it. If it happens again, then I'd make a trip to the gynecologist. My periods are usually very regular, though they can fluctuate a day or two. However, there seems like once or twice a year (usually once in the summer and once in the winter), my period will come a week early or a week late. Sometimes traveling or seasons seem to affect it. Also, when I'm around other females frequently, I'll notice that my cycle will change. Women tend to "sync" when they're around each other, which can affect your periods.

There's all kinds of crazy factors that affect your period other than stress. Some, you might not even think about!