Periods are the end of a cycle that begins with ovulation -- ovulating is the starting event of your month and the period follows that.
Sometimes women don't ovulate. Stress can cause this, or possibly diet (lack of body fat can do it), and sometimes they are influenced by the pheremones of a new roommate and their cycle begins to swing around to match hers, and sometimes they just don't. If you don't ovulate one month, or your ovulation time of the month is shifting, your body might not bother to put on a uterine lining that month, or might not have much of one by the time it decides to shed it in a period.
You can also get these symptoms if you get an ovarian cyst. Once when I had one, my doctor asked "Was your last period kind of odd?" and it was.
In your shoes I wouldn't worry, but would kind of keep an eye on everything next time your period is due to see if you're back to normal.