I have missed my period and i have been bleeding verry verry lighly (on and off) for about two - three weeks now. you know the bleeding like when you first discover you have your period? like that. then it stops. then it happens again. what's wrong?

yes i have unprotected sex all the time, and yes i have checked pregnancy tests, 4 to be exact, and they all turn out negative. ( i did one test every week)

what could this be?