What Causes Irregular Periods?

i m not having my periods since i m having sex with my husband on the first week of february and taken i pill as contraceptive? could u plz tell me wat should i do, we don't want to bear child right now, my periods date is 17 and till now i m not having it. i m having a feel of it , last month also my bleeding was very less than normal, wat should i do?plzzz tell me.