Hi Andrea,
From my experience, you carry on as if you have a 28 day cycle. So keep taking the progesterone until day 26. It’s not unusual to have a period earlier or later than expected. I assume, because you are on this regime you are peri menopausal? During peri, our own hormones fluctuate wildly in the background. Some months I had 2 periods, other months I barely had one at all. It can be a rollercoaster ride.
Stick to the 28 day regime you have been prescribed. It can take a while for your body to adjust.
Good luck and welcome btw.