What Does Membrane Like Discharge Indicate During Periods?

Last month my period did not come. This month my period came but it was very light and lasted barely two days. But my period this month the cramps were unbearable! When i went to change my tampon this long membrane looking thing came out with the tampon. Out of curiosity i stretched it out between my fingers and it was almost see through and it had this dark thick line curled inside of it. What is this thing that came out of me?