Charli - just typed "what is amnion" in google and this was the clearest description i could find for you:
"A transparent sac, filled with colorless fluid, surrounding the embryo; the amnion and amniotic fluid protect the developing embryo from shock and permit it to exercise".
I gather from that description that the amnion is not the actual fluid therefore, it's the name for the membrane which forms the sac that holds the fluid.
I tried to delve a bit further, i'm not a doc, so this is just my understanding of some of the medical stuff i read.
From what i can gather there are 2 membranes that cover the fetus. Each of these membranes surround cavaties/spaces which are filled with fluid. The 'amnion' is the inner membrane sac, and the 'chorion' is the outer membrane sac.
I really couldn't find anything which indicated if an enlarged amnion and extra membrane were indicators of miscarriage, or indeed were considered cause for concern. There seemed to be very little on this.
I think it is vital that you ask your doc for a more detailed explanation of the what these are and the implications, in plain English. I don't think it is at all acceptable for doctors to diagnose things and put a name to them but then fail to explain them to you and leave you to go away worrying yourself senseless. It's disgraceful. You have every right to demand that they do this. I am horrified at some of the stuff the women on here have said about docs being rubbish and failing to give them correct information and support.
Anyway, on a positive note, I think the fact that they've found a heartbeat at only 6 weeks is heartening (I was told it's generally 7 before they can really tell), and that you've gone past the point that you miscarried last time. Also it could be that the discharge was just implamentation bleeding.
I hope that helps a little. Don't be afraid to ask LOTS of questions, it's your body. Good luck x