What Could Cause Lower Abdominal Pain Despite My Next Period Being Due In 3-4 Days?

Hi, I was on Duphaston twice a from day 16 to 26 as per doctor prescribtion. I got lower abdominal pain and I ran pregnancy test on 24th day from periods and it turned out positive. But Later I checked it was negative ,why is it so?my next periods will start in 3-4days but I am still having lower abdominal pain.why this happeneg?