What Causes Delay In Periods?

hi ...we had sex last month today it has been exactly a month .I took pill to stop pregnancy available in market .But still i have not got periods . As a result i am really worried to know weather i am pregnant or what ?? it has been exact 30 days since then. At the time of sex i was having periods. And i think it is a safe time to have sex . it has been 30 days now but not have got periods . Should i wait more for the period ?? Can it be late due to pill ??? When should i take the next step ? and what is the next step ?? Please let me know i am very worried .