HI! Last Week, I Had A Painful Period, Tummy Cramps,

HI! Last week, I had a painful period, tummy cramps, back pain as well and this was all day.Before we were closing around 5pm, blood clot came out from me and big white tissue.The doctor told me was just a blood clot but I asked if it is a blood clot how come its white, very weird look like,and pregnancy test-negative. 2months ago, June 17 I had my (prevora?)shot, coz of my heavy period last long for 6weeks.Still don't know what was that white tissue came out from me and its quite big.Hope you can help me for this issue.