What Causes Delay In Menstruation Other Than Pregnancy?

hi i am 18 years old and to my knowledge healthy
i had my last menstral period on the 20th of june 2010 and on the 7th of july 2010 i had sex with my boyfriend and we used the withdrawl method 3 days after i got brown like very light periods and when i went to the bathroom i had very light pink spotting on the tissue the brown lasted for 2 days ( this happened exactly 1 week before i was due to have my periods and the only time i had sex during this month is on the date i said above)
now my period never came and now i am 7 days late still having cramps and sometimes backache but no periods did quite a few pregnancy tests but all came back negative
what could be the problem pls