When I first went on estridiol a few years back, I found myself in a deep mental fog. Not only was I insanely tired, I couldn't concentrate on anything and work became nearly impossible. It got so bad that after a few days I left a voicemail for my doctor. But as it was a Friday I didn't hear from her. So, I stopped taking the drugs and within an hour or two the fog lifted. It was like I had woken up and was finally "me" again.
Fast forward a couple years and the dysphoria came back with a vengeance. When I went to the doctor again, I mentioned the fogginess but she said she had never heard of something like that from any of her patients. And this is a doctor at Callen-Lorde who deals with HRT everyday.
I started the drugs again and it was fine. The fogginess wasn't there. When I explained this to my doctor she said it was likely something unrelated. I couldn't imagine how. Like you, my fogginess started and stopped with HRT.
So either way, something to discuss with your doctor, but you should know that it might not be a permanent effect. Also, it's just really good to know that I wasn't the only one who felt something like that! So thank you for posting this!