hi..this is the first time i do this but im really worried.. because its the first month that im on injected birthcontrol.. and its great but i got my period normal and everything but its stoped on the the 25 and im was thinking its stoped so i had sex with my boyfriend and guess what.. i notice a dark spotting on my undies.. its gotten lighter but im kinda sacred.. is this normal.. at all. please some one help me..