What Causes Discharge Of Milky Tissue Like Thing With Bleeding?

i have been taking norethisterone for 15days to prevent irregular bleeding.3days after,the bleeding started,just like to doctor said it would.what i am worried about is than on the first day some sort of milky tissue looking string cca 20cm long,with a buble on one side and fell out of my vagina.what was that?should i worry?