WTF is THAT?! Tissue?!

I need help/advice.

Warning: details follow.

Oh, man. So late last night I my period started. This is the first one since the retrieval, so it's usually pretty nasty. It was just a bit of bleeding last night. Today was really the first day.

Here's the freak out: I was in the bathroom and I looked down and on my pad was a piece of tissue. Literally, I looked at it and went "oh my god". So I had to examine it. It wasn't a clot - I do get clots, especially after a retrieval, and I know what those are. This had some blood on it, but it was skin like.

So some googling around suggests that this was normal endometrial tissue and that These Things Happen and that I Shouldn't Be Alarmed. Has anyone had this happen to them? It freaks me out to have fleshy parts of my inside ending up outside....