What Causes Fleshy Substance During Menstruation?

I found something in my period too, i found what looks like a sort of membrane it looked like the stuff that comes out of ur mouth when u brush too much, like fleshy. What is it? Im 22 years old i've never been pregnant, i was on the pill a few months back but have stopped because i don't ned to be on it anymore (i've stopped having sex).