Hi Anja
we all assimilate medication at different rates and you might find that when you slap a new patch on the old patch still had a decent dose left in it, so there is an overlap ( rather than the dose in your body decreasing due to the age of the patch) and you might be getting a bigger dose on day 1 of the patch change.
I had gut problems when I started on HRT and tried a variety of HRT medications including patches. In the end I tried low dose Premique. It stopped my hot flushes and I didn't have any gut problems. I also found that after a year on the Low dose, I could swap to the ordinary dose without any of the side effects I had suffered in the beginning.
May be worth a try, or change your patches a day later and see what happens
Good Luck
Anne