What Causes Painful Periods And Discharge Of Sac Of Tissues?

This morning I woke up and was going to change my pad, I am on a 5 day break from the pill and when I took the pad of there was some sort of sac on it at first I thought it was just clotted blood but it was all attached and I could see like a pale colour like epilthilium or something. I am not pregnant, but I dont know what it could be? And my period is unnaturally more painful that usual