Talk to Gynecologist on Missed Periods

I was advised to take Duphaston from day 15 of my cycle for 10 days (twice daily) when I plan on conceiving, after a miscarriage in December 2019. I have been taking it for 2 months now. My cycle was regular in june (28 days). July cycle has been missed it has been 25 days now. I have never missed a cycle after taking Duphaston. Can you please suggest what might be wrong?

I had a history of irregular cycles due to high prolactins which was treated earlier and I even coceived. My cycles were regular since then even after the miscarriage.