Can My lining decrease without a period?


So my story is a little different in that I do not have ovaries anymore - there for I do not make any hormones of my own. I went the beginning of August to have my baseline ultrasound to start my round for a FET. (I normally take HRT so I am not going into menopause) and my lining was a 15!!! that was after having a “period” which was just spotting but normal for me since my surgery. after waiting a month and taking 10 days of provera still no period by my fertility clinic now wants me to go in for another baseline ultrasound - they say if it is thin then they will go ahead and start my FET cycle. I have had NO estrogen for over a month the only hormones I have had all month long were the 10 days of provera. I am having hot flashes and all of my other menopause symptoms back. but I have had NO BLEEDING! NONE! Not even a little bit of spotting. So my question is… can my lining change?? Is it possible my lining thinned over this last month even if I didn’t bleed? I really don’t want to waste money to go in for an ultrasound again if it isn’t even possible. :frowning: