i wouldn't be too concerned about mood swings from the morning after pill. basically you are tricking your body into a "new" cycle and it's normal to experience some moodiness from a fairly large dose of hormones. i would agree with AAlexander though, that if it is an ongoing problem then perhaps a visit to your physician would be the best route to take thereafter. wait it out for a week or so though, you're body takes a while to re-establish itself after that kind of dosage. i have known some gf's to have gone through the same symptom and with a little time to get back to their own regular "clock" they have been right back to tickety boo!
Anytime you mess with the body's natural hormones you can bring on depression and mood swings. If your depression lasts more than a couple weeks, go see your doctor for an evaluation to see what could be the cause, and how severe it is.