What Causes Delayed Period After Having Protected Sex While HPT Is Negative?

i want to ask about menstrual problem. i am 20 year old girl. i had intercourse with my boyfriend previous month twice n both the time i had pill that too continuously n i had my periods that month but next month i again had intercouse n used protection but i am not having my periods. i even checked for pregnancy but it was negative.in middle of month i had some bleeding but it stopped in half or more hour that happened after few days of intercourse . please tell me if i should take any medicine or what should i do. i have already crossed my period date which should be on 31st.