What Causes Light Menstrual Bleeding Along With Back Pain?

I got my period on Sunday it was very light all Sunday and on Monday it was light and it would come and go light then Tuesday it was some light and mild all day with cramps and back pain on wed it was light and mild then at night it was mild with a few blood clots and today it's light all day but my periods only last 3 days heavy then goes away what does that mean