Perhaps it is your body adapting to the progesterone.
When you first start it, among other functions it antagonises mineralocorticoid receptors and has a diuretic effect.
After a few days your body catches up and increases aldosterone secretion to help restore fluid and sodium balance. Sometimes this transiently overshoots, leading to fluid retention which can cause that PMSy feeling, then it rebalances.
I am aware of this because I am on mineralocorticoid replacement and when I was having natural menstrual cycles the luteal phase increase in progesterone made it difficult to titrate my fludrocortisone.
The solution to this is to achieve stable hormone levels rather than seesawing up and down with progesterone.
For me this means continuous intake of a combined pill for menstrual suppression.
As you are postmenopausal this could be achieved by taking a lower dose of progesterone every day rather than cycling.