It's your body and metabolism finally settling down after starting HRT. These biochemical changes impact a very delicate balance of hormones between your pituitary and all the other glands in your body. It takes several weeks for the initial impact of HRT to be felt, then it's many, many more weeks before things finally adjust and settle down. This is when HRT begins working and you notice the subtle physical changes. From that starting point, it's a long, slow process of small gains, then stability, then gains, then stability. It's your female puberty

and it should require from two to four (or more) years to complete.
I'm glad your doctor checked your blood levels, but did they also check your T level? The longer you stay on HRT, the less often you should be "surprised" with that issue.