Maybe the progestogen? I never notice my oestrogen patch change at all - nor when it "needs" changing - because they are designed to last a week even though the prescription says change twice a week ( according to info I read) - this is to ensure they don't "run out" and keeps levels much more stable.
However whenever I start the progestogen part of my cycle ( I am on long cycle HRT) then I no start to get odd twinges - definitely uterine - not bad pains - but I can "feel" the changes in the uterus lining beginning to take place (probs because I'm old and my womb is old

).
As the Femseven is kept on for a week - it could be there is a bit of a decline in levels (progestogen) after this time and you notice it starting to take effec?. Maybe even women notice the change in uterus lining when oestrogen is added although can't say I have?
Also do you have fibroids or anything like that, that you know of?
Hurdity x