What Causes Delay In Periods And Spotting?

hi on the 20th of june 2010 i had my regular period that lastes 4 days on the 7th of july 2010 i had sex with my boyfriend and we used the withdrawl method on the 10th of july i started having like a very light brown period which lasted 2 days and when i went to the bathroom and wiped with the tissue i had very light pink spot this brown light period came a week exactly before my period which never came and now i am 7 days late did quite a few hpt and all came negative
any idea of what is happening pls