What Causes White And Brown Discharge After Periods?

I had my period on the 13th it ended on the 16 and sat I noticed a white thick discharge well today I had another discharge and it was white and had a little bit of brown in it. I have been getting cramps that last 2-5 min then goe away well about four hours ago I started suffering lower back pain and it is about a 8 on the pain chart. What could this be?