I sometimes get mild cramping or should I say aching, when on the progesterone part of HRT - especially on a long cycle.
By the way matildamouse/Sunnydays - don't worry about the progestogen "eating away" at your lining! It doesn't really do this as such (although in a way norehtisterone can - see below)! Well it seems to act differently on sequential and continuous HRT. With sequential HRT - when you are on the progestogen part, the progestogen converts the lining from proliferative ( growing) as it does on oestrogen only - to "secretory" (changes in structure ready for nurturing the embryo). The different layers of cells change completely in structure. When you stop the progesterone this signals the lining to be shed and the usual cramping is part of the mechanism whereby the uterus expels the old lining. I'm not sure why we feel cramps during the secretory stage but presumably some women can actually sense the changes going on.
When you are on continuous combined HRT the progestogen prevents the lining from building up at all and so it stays thin. I have read that it interferes with the oestrogen receptors preventing the oestrogen from stimulating the linging to grow. On looking this up (quite hard to find precise info and all gets very detailed and technical) some studies show that the lining can contain proliferative and secretory cells at the same time (on conti HRT). I am not certain as to the mechanism where norethisterone actually thins the lining when given to women who have a thickened lining, but it is a very potent progestogen in terms of its action on the uterus, compared to progesterone.
I got carried away a bit there but hope it helps - sorry I've not got references to hand! You can probably tell I only have a rudimentary knowledge of all of this from memory and don't have time in the summer to research/revise stuff!
Hurdity x