What Causes Longer And Thicker Discharge From Vagina During Periods?

i just got my period yesterday and right now when i went to the bathroom to change my tampon i noticed this thick thing coming out of my vagina, as usual i thought it was just a normal blood clot but it was longer and thicker. i placed it on a plate to examine it and the sak like material layed flat stretches out to about an inch and a half in diameter with a grey thing in the middle. what is this?!