Had a thick tissue discharge during third day of period. Also experienced sharp pain. What does this mean?

Endometrium. What actually happens during a menstrual period is that the inner lining of the uterus that has been built up during the first half of the cycle in preparation for the implantation of a fertilized egg--now that there is no pregnancy--is being discharged from the uterus, hence the tissue clots. The sharp pain reflects the work of the uterus to discharge the tissue.