Stress causes a hormone called cortisol. Cortisol can interfere with your other hormones and cause you to miss an occasional period or have one late. When the stressing factors are gone, you should balance back out if this is the case.
Have you had an increase in physical activity? Strenuous exercise causes testosterone levels to rise which could cause you to stop having periods. Being overweight can also cause this. I would not worry about one missed since you are sure you are not pregnant. I would try to get back on the iron though. If this does not resolve, go to the doctor and have them test for possible ovarian cysts/endometriosis. This can generally be fixed by estrogen birth control medication.