Ok...soooo 13 mls of fluid in your Pouch of Douglas (which is the area between the uterus and rectum in females) which is likely caused by EPU.
There was no evidence of Arteriovenous Malformation in your pelvis which is good though the tachycardic episode/drop in BP wouldn't have been nice on top of all the bleeding
Your lining being 7mm means that you did indeed have a lining that was consistent of where you were in your Cycle/transfer. Having too thick a lining (which is called Endometrium Hyperplasia) is a risk factor for cancer of our "Lady Bits" though it doesn't cause it outright, so I daresay the Gynae was looking in that direction as a possible cause for your bleeding episode and the fact your lining was within the normal limits ruled that out
I'm still putting my money on what I said it was in my earlier post (having mind blank atm!!!)