When not caused by pregnancy, menstrual irregularities are usually a sign of a larger condition or problem. There are many conditions that can cause menstrual irregularities such as PCOS, POF, uterine fibroids and endometriosis.
THe side effects of the morning after pill may include
* breast tenderness
* irregular bleeding
If your menstrual cycle continues to be irregular after a couple of months, I'd suggest that you schedule a visit to see your gynecologist. Otherwise, I don't think that the early bleeding is anything to worry about.