Pls Doctor I Don't Understand Cos I Was Given Dufaston

Pls Doctor i don't understand cos I was given dufaston drugs last month and my period came out a day after,my gynecologist layer gave me sinopol for 15 days and I have missed my period,having serious back pain, cramps on my stomach, weak etc bt she told me to repeat the dufaston drugs if my period didn't come on is own bt if it comes, dat I should take it. I did a pregnancy test bt negative. Pls explain to me doctr. Thanks