What Causes Cramping After Periods?

Hi my name is Emma. I have my tubes tied and i know i can't get pregnant but last month my period came down always normal . i get my period for 3 to 4 days and after the fourt day I wait another three days for cleanse and it brown. The brown was gone then I had a white discharge with lite pink one morning. Ever since I have been cramping on and off. What does this mean?