How To Postpone Periods?

Hi doctor, i have pcod problem. but after my second delivery i am getting periods. but some inorder manner. like sometimes before one week and sometimes after. i am feeding my 11 months baby daily 3 times. my period date is tomorrow. i have to postpone my period till 19th. what tablet can i take? and what abt side effects and when can i get back my period . and at what interval i should take the pill?