If the blood stops and you see the white thread like discharge, you are pure and must pray! The discharge you see days afterwards has no effect on your purity. Simply purify yourself from it and make wudhu and pray.

I am 39 years of age previously my period was 6 days, now the blood stops after three days and sometimes white fluid also comes then after 3 or 4 days again I experience brownish discharge (liquid & solids) for 3 or 4 days, should I count all this time as my period? when should I consider myself pure for Salah, Fasting etc.?