Hello. I Was Prescribed To Take Duphaston For 3 Months;

Hello. I was prescribed to take Duphaston for 3 months; for 10 days twice daily. Reason - to regulate my cycle. I'm 45 and having irregularities (too short periods betwen cycles). The doctor said i should get my period 2-3 days after finishing the Duphaston. It happened like this the first month, but now i got my period during the 8th day of taking the pills. Should i stop taking the pills now, or shpuld i finish the 10 days course? Also, why did i get my period during taking the pills? Thank you!