What Causes Spotting Before Periods After Taking Duphaston?

Hi, I have been diagnose with hyperprolactinaemia. I have been taking Proctinol and now the prolactin levels are fine. i am trying to conceive and have been advised Siphene 100mg from D2 - D5. i did not conceive through this during my last cycle.i took dupahston from D16 for 14days. I did not get get my period on the scheduled day however i did spot heavily for 2 days a brown discharge and started my periods 4 days after that stopped. why did this happen?