Massive Luteal Phase Bleeding post-FET

Has anyone else had massive bleeding in their luteal phase that wasn't their period? I had a FET (5dt) last Sunday and 4 days later started to bleed a little in the morning and continued at maybe 'medium-light-day' flow for the rest of the day until evening and then overnight moved into heavy flow after the evening Endometrin dose. I had spoken to my nurse early in the day and been reassured that some bleeding can be normal, that since I was on Endometrin, it couldn't be my period and besides was totally the wrong time for it. I called again in the morning, sure that somehow both my natural progesterone and Endometrin had failed to prevent my period coming. They had me in for an ultrasound and found my lining intact, thick, smooth surface, no pools of blood as they typically see during the menstrual period, no source of bleeding and a healthy corpus luteum on each ovary that should be pumping out progesterone. My RE decided to switch me to PIO, and sent me home. I bled heavily enough the rest of the day yesterday that I kept wondering at what point I was supposed to call the hospital. Then around 4, I passed a clot the size of a mandarin orange and after that, the bleeding has slowed to more like heavy or medium-heavy flow. It did seem to worsen each time I put in an Endometrin, and got really bad after the ultrasound. My RE seems baffled. My nurse coordinator has never seen anything like it in 11 years. I'm 99% sure that my chances of being pregnant are nill (negative HPTs as well) but I'm concerned about the bleeding. Something somewhat similar happened about a week after my SHG last summer where I bled heavily and passed large clots for a week - doctor could find no source and said it wasn't my period and he was right, as it came about a week after the unexplained bleeding stopped. That doctor basically said that they probably irritated my uterus with the SHG and it was just bleeding. Where can that much blood and tissue come from if not the uterine lining? Could these be related? Has anyone else had something like that? This was my only normal embryo, so I'm headed back to square one I assume, and terrified of this happening in my next transfer, especially since we have no idea what caused it and therefor can't prevent it. Both times I could tell the blood loss affected my energy. In the previous one it took about a week for me to be able to complete my normal morning workout again.