I've only been on HRT since Friday, and as of a few hours ago I noticed that my nipples are feeling particularly sensitive.  It seems kind of early, especially since I haven't noticed any other changes, not even emotional ones (apart from the excitement of starting HRT!).  Thoughts on whether I'm experiencing physical changes or just psychosomatic ones?

It's definitely possible. In the first week on E only, a small dose, I was having changes. First day I got a migraine, on the 3rd or 4th day my scent changed and within a week my skin felt markedly softer. Nipples became constantly erect and more sensitive in short order as well.

It's definitely possible; I had visible breast growth and breast buds within 5 days. Hang on, it's going to be a wild ride.

I just find this all so encouraging and helpful! Thanks for sharing your experiences!