think my dr is lying

Is anyone on her a ultra sound tech- or work in a dr/OB office-  I have a question, I think my dr is lying to me.   Long story short- had miscarriage- had a D&C, 2 days later I was back in the hospital- they said the ultrasound shows I still had tissue in the uterus- & I had to take "the abortion pill" to clean it out... I saw my dr yesterday & he's claiming he thinks it was a clot & blood was blocked.  & that a ultrasound can't tell the difference if it's a clot or tissue.... Is this true?