What Caused Light Spotting, Cramping, Back Pains And Clotting?

hi there.. i was having what i thought was my period from the 18th to the 21st of this month but it was very light and abit brownish and only lasted four days..i had cramps and slight back pains which im still getting and also during the four days there was tiny little clots of blood in the toilet and on the pad as well. so what do you think it can be..