An irregular period can come from things outside of your diet. Stress, change in routine, weight loss, being underweight and hormone imbalances can all impact the cycle of your period. If this is a persistent issue I would recommend discussing this with a gynecologist who can better understand your cycle and hormone balance.
Choosing foods rich in phytoestrogens may help. These include legumes (particularly organic soybeans - tofu), flaxseed, other beans (lima, split peas), and alfalfa sprouts. Also, the herbs black cohosh, chaste berry, ginseng and red clover have also been reported as effective in menopause and PMS symptoms, but you should have the advice of an herbalist or naturopathic doctor before you add this to your diet. Add a few of the phytoestrogen-rich foods in with some leafy greens, banana, avocado, berries, grapes, nuts and seeds (sunflower and pumpkin)!
Check out some info on Chaste Tree Berries here: http://www.nutriliving.com/ingredients/chaste-tree-berries