What Causes Low Menstrual Flow?

So I think I m getin my period I did bleed little bit but not much has come I have been stressed out but does that mean my period is comin because today I wiped myself and was blood nothing bad but is my period about to happen is this normal? I m 21 I do not take birthcontrol pills yet but I never have this problem Cus once I get my period I bleed a lot on lik the first day I do feel little cramps but if like to know if this mean my period is about to happen!